Cache parsed markup for variables in LRU

Here's the current benchmark results for liquid/master on my box:

```
                   user     system      total        real
parse:         2.090000   0.000000   2.090000 (  2.095237)
parse & run:   4.620000   0.020000   4.640000 (  4.642556)
```

Here is the cpu profile:
```
==================================
  Mode: cpu(1000)
  Samples: 1054 (1.13% miss rate)
  GC: 82 (7.78%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
       154  (14.6%)         100   (9.5%)     Liquid::Variable#lax_parse
       924  (87.7%)          88   (8.3%)     Liquid::Block#parse
       178  (16.9%)          77   (7.3%)     Liquid::Context#variable
       252  (23.9%)          74   (7.0%)     Liquid::Context#resolve
        66   (6.3%)          66   (6.3%)     Liquid::Template#tokenize
        54   (5.1%)          54   (5.1%)     block in Liquid::Variable#lax_parse
       199  (18.9%)          39   (3.7%)     Liquid::Variable#initialize
        43   (4.1%)          36   (3.4%)     block in Liquid::Context#variable
        28   (2.7%)          28   (2.7%)     Liquid::Context#has_interrupt?
        28   (2.7%)          28   (2.7%)     Liquid::Context#lookup_and_evaluate
      1149 (109.0%)          25   (2.4%)     block in Liquid::Block#render_all
        24   (2.3%)          24   (2.3%)     block in Liquid::Context#find_variable
        27   (2.6%)          23   (2.2%)     Liquid::If#lax_parse
      1171 (111.1%)          22   (2.1%)     Liquid::Block#render_all
        22   (2.1%)          22   (2.1%)     Liquid::For#lax_parse
        17   (1.6%)          17   (1.6%)     Liquid::Context#increment_used_resources
        16   (1.5%)          16   (1.5%)     Liquid::StandardFilters#strip_html
        11   (1.0%)          11   (1.0%)     Liquid::Context#resource_limits_reached?
       307  (29.1%)          11   (1.0%)     Liquid::Variable#render
        10   (0.9%)          10   (0.9%)     Liquid::StandardFilters#truncatewords
```

And the object allocation profile:
```
==================================
  Mode: object(1)
  Samples: 9639300 (0.00% miss rate)
  GC: 0 (0.00%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
   1947200  (20.2%)     1462300  (15.2%)     Liquid::Context#variable
   2185900  (22.7%)     1433400  (14.9%)     Liquid::Variable#lax_parse
    833800   (8.7%)      833800   (8.7%)     Liquid::Context#has_interrupt?
  10194700 (105.8%)      806100   (8.4%)     Liquid::Block#parse
    752500   (7.8%)      752500   (7.8%)     block in Liquid::Variable#lax_parse
   2944500  (30.5%)      521600   (5.4%)     Liquid::Block#create_variable
    438600   (4.6%)      438600   (4.6%)     Liquid::Template#tokenize
    480300   (5.0%)      433300   (4.5%)     Liquid::Context#find_variable
   2368300  (24.6%)      421100   (4.4%)     Liquid::Context#resolve
   3075100  (31.9%)      300600   (3.1%)     Liquid::Variable#render
   1075400  (11.2%)      295500   (3.1%)     block in Liquid::Variable#render
    289100   (3.0%)      289100   (3.0%)     Liquid::If#lax_parse
   2422900  (25.1%)      238800   (2.5%)     block in Liquid::Block#create_variable
    204800   (2.1%)      204800   (2.1%)     Liquid::StandardFilters#truncatewords
    146100   (1.5%)      143500   (1.5%)     Liquid::For#lax_parse
  10927100 (113.4%)      108700   (1.1%)     Liquid::Block#render_all
    600600   (6.2%)       98500   (1.0%)     Liquid::Context#invoke
     70700   (0.7%)       70700   (0.7%)     Liquid::Block#block_delimiter
     48300   (0.5%)       48000   (0.5%)     Liquid::Context#initialize
  10764400 (111.7%)       47600   (0.5%)     Liquid::Tag.parse
```

Liquid::Context#variable is responsible for the most allocations and is the third most prominent CPU hog. In addition, live profiles of Shopify identify it as allocating ~4.5% of the total Ruby objects (!) and taking ~1.5-2% of the CPU. In short, we call this *a lot* and it's reasonably expensive.

Nearly all the objects and good chunk of the CPU comes down to this line:

```
parts = markup.scan(VariableParser)
```

All this does is take a string and break it up into an array using . and [] as delimiters. There's no real way to remove the allocation here -- doing this via regexp is going to create objects. Since Liquid templates are likely to be seeing the same variables over and over and over again, I introduced an LRU cache for the results of markup.scan(...). That way, the bits which are done repeatedly will be fast and allocate few objects while the unique snowflakes will be no worse than before.

The results are pretty good... benchmark:

```
                   user     system      total        real
parse:         2.080000   0.000000   2.080000 (  2.081288)
parse & run:   4.480000   0.000000   4.480000 (  4.487226)
```

cpu profile:
```
==================================
  Mode: cpu(1000)
  Samples: 1057 (1.31% miss rate)
  GC: 71 (6.72%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
       183  (17.3%)         119  (11.3%)     Liquid::Variable#lax_parse
       971  (91.9%)          92   (8.7%)     Liquid::Block#parse
       217  (20.5%)          84   (7.9%)     Liquid::Context#resolve
        64   (6.1%)          64   (6.1%)     block in Liquid::Variable#lax_parse
        63   (6.0%)          63   (6.0%)     Liquid::Template#tokenize
        34   (3.2%)          34   (3.2%)     LruRedux::Cache#[]
       225  (21.3%)          32   (3.0%)     Liquid::Variable#initialize
        28   (2.6%)          28   (2.6%)     Liquid::Context#has_interrupt?
      1069 (101.1%)          25   (2.4%)     block in Liquid::Block#render_all
        33   (3.1%)          24   (2.3%)     block in Liquid::Context#variable
        24   (2.3%)          24   (2.3%)     Liquid::Context#lookup_and_evaluate
        22   (2.1%)          22   (2.1%)     Liquid::Context#increment_used_resources
        28   (2.6%)          21   (2.0%)     Liquid::If#lax_parse
        20   (1.9%)          20   (1.9%)     block in Liquid::Context#find_variable
        20   (1.9%)          19   (1.8%)     Liquid::For#lax_parse
        16   (1.5%)          16   (1.5%)     Liquid::Context#resource_limits_reached?
       133  (12.6%)          15   (1.4%)     Liquid::Context#variable
        15   (1.4%)          15   (1.4%)     Liquid::Tag#initialize
       282  (26.7%)          15   (1.4%)     Liquid::Variable#render
        14   (1.3%)          14   (1.3%)     Liquid::Template.error_mode
```

object profile:
```
==================================
  Mode: object(1)
  Samples: 8811300 (0.00% miss rate)
  GC: 0 (0.00%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
   2185900  (24.8%)     1433400  (16.3%)     Liquid::Variable#lax_parse
    833800   (9.5%)      833800   (9.5%)     Liquid::Context#has_interrupt?
  10194700 (115.7%)      806100   (9.1%)     Liquid::Block#parse
    752500   (8.5%)      752500   (8.5%)     block in Liquid::Variable#lax_parse
   2944500  (33.4%)      521600   (5.9%)     Liquid::Block#create_variable
    438600   (5.0%)      438600   (5.0%)     Liquid::Template#tokenize
    480300   (5.5%)      433300   (4.9%)     Liquid::Context#find_variable
   1540300  (17.5%)      421100   (4.8%)     Liquid::Context#resolve
    321900   (3.7%)      321900   (3.7%)     LruRedux::Cache#[]
   1119200  (12.7%)      312400   (3.5%)     Liquid::Context#variable
   2434900  (27.6%)      300600   (3.4%)     Liquid::Variable#render
   1058600  (12.0%)      295500   (3.4%)     block in Liquid::Variable#render
    289100   (3.3%)      289100   (3.3%)     Liquid::If#lax_parse
   2422900  (27.5%)      238800   (2.7%)     block in Liquid::Block#create_variable
    204800   (2.3%)      204800   (2.3%)     Liquid::StandardFilters#truncatewords
    146100   (1.7%)      143500   (1.6%)     Liquid::For#lax_parse
   8876400 (100.7%)      108700   (1.2%)     Liquid::Block#render_all
    600600   (6.8%)       98500   (1.1%)     Liquid::Context#invoke
     70700   (0.8%)       70700   (0.8%)     Liquid::Block#block_delimiter
     48300   (0.5%)       48000   (0.5%)     Liquid::Context#initialize
```

So, that's 828,000 less objects and a ~150 ms difference in cumulative CPU time.

There's a couple things I wasn't sure of here:

* Is 10,000 a reasonable size for the LRU?
* The LRU cache is not thread safe (I don't see any reason why it would need the extra overhead....) -- should it be?

@Shopify/liquid
For thread safety, I know it's not safe (see description). I just didn't kniw if liquid made any promises re: thread safety.

I'll use the thread safe version but that will likely kill any CPU gain (have to benchmark again, but I did try it and seem to remember that's how it fared).