Optimize block parsing -- don't recreate delimiter, use strings instead of regex

Block parsing is a very hot path in both the Liquid benchmark and Shopify itself. 

Two optimizations in this change:
* Created the block_delimiter in the constructor, so it does not have to be recreated on every parsing pass (saves useless object creation)
* Changed parsing to use string comparison instead of regexp -- the regexps being used were simply "\A\{\{" and "\A\{\%". That's something String.start_with? can handle much faster and without creating temporary objects.

Benchmark of master (best of 3 runs):
```
                   user     system      total        real
parse:         2.120000   0.000000   2.120000 (  2.119865)
parse & run:   4.410000   0.020000   4.430000 (  4.422620)
```

Object allocation profile for master:
```
==================================
  Mode: object(1)
  Samples: 8171200 (0.00% miss rate)
  GC: 0 (0.00%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
   2185900  (26.8%)     1433400  (17.5%)     Liquid::Variable#lax_parse
  10194700 (124.8%)      806100   (9.9%)     Liquid::Block#parse
    752500   (9.2%)      752500   (9.2%)     block in Liquid::Variable#lax_parse
    625200   (7.7%)      625200   (7.7%)     Liquid::Context#variable_parse
   2944500  (36.0%)      521600   (6.4%)     Liquid::Block#create_variable
    438600   (5.4%)      438600   (5.4%)     Liquid::Template#tokenize
    480300   (5.9%)      433300   (5.3%)     Liquid::Context#find_variable
   1716000  (21.0%)      421100   (5.2%)     Liquid::Context#resolve
   2521700  (30.9%)      300600   (3.7%)     Liquid::Variable#render
   1053600  (12.9%)      295500   (3.6%)     block in Liquid::Variable#render
    289100   (3.5%)      289100   (3.5%)     Liquid::If#lax_parse
   2422900  (29.7%)      238800   (2.9%)     block in Liquid::Block#create_variable
    204800   (2.5%)      204800   (2.5%)     Liquid::StandardFilters#truncatewords
    146100   (1.8%)      143500   (1.8%)     Liquid::For#lax_parse
    131300   (1.6%)      131300   (1.6%)     block in Liquid::Context#variable
   7068300  (86.5%)      108700   (1.3%)     Liquid::Block#render_all
    600600   (7.4%)       98500   (1.2%)     Liquid::Context#invoke
     70700   (0.9%)       70700   (0.9%)     Liquid::Block#block_delimiter
     66300   (0.8%)       66000   (0.8%)     Liquid::Context#initialize
    683300   (8.4%)       58100   (0.7%)     block in Liquid::Context#initialize
```

Benchmark of this PR (best of 3 runs):
```
                   user     system      total        real
parse:         1.810000   0.000000   1.810000 (  1.809644)
parse & run:   4.290000   0.000000   4.290000 (  4.294896)

```

Object allocation profile for this PR:
```
==================================
  Mode: object(1)
  Samples: 7880000 (0.00% miss rate)
  GC: 0 (0.00%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
   2185900  (27.7%)     1433400  (18.2%)     Liquid::Variable#lax_parse
    752500   (9.5%)      752500   (9.5%)     block in Liquid::Variable#lax_parse
    625200   (7.9%)      625200   (7.9%)     Liquid::Context#variable_parse
   3019900  (38.3%)      597000   (7.6%)     Liquid::Block#create_variable
   9428200 (119.6%)      474300   (6.0%)     Liquid::Block#parse
    438600   (5.6%)      438600   (5.6%)     Liquid::Template#tokenize
    480300   (6.1%)      433300   (5.5%)     Liquid::Context#find_variable
   1716000  (21.8%)      421100   (5.3%)     Liquid::Context#resolve
   2521700  (32.0%)      300600   (3.8%)     Liquid::Variable#render
   1053600  (13.4%)      295500   (3.8%)     block in Liquid::Variable#render
    289100   (3.7%)      289100   (3.7%)     Liquid::If#lax_parse
   2422900  (30.7%)      238800   (3.0%)     block in Liquid::Block#create_variable
    204800   (2.6%)      204800   (2.6%)     Liquid::StandardFilters#truncatewords
    146100   (1.9%)      143500   (1.8%)     Liquid::For#lax_parse
    131300   (1.7%)      131300   (1.7%)     block in Liquid::Context#variable
   7068300  (89.7%)      108700   (1.4%)     Liquid::Block#render_all
    600600   (7.6%)       98500   (1.2%)     Liquid::Context#invoke
     66300   (0.8%)       66000   (0.8%)     Liquid::Context#initialize
    683300   (8.7%)       58100   (0.7%)     block in Liquid::Context#initialize
     53000   (0.7%)       53000   (0.7%)     Liquid::Block#initialize
```

Down 291,200 objects and ~150ms.

@Shopify/liquid 

Could we memoize this for the class instead of on the instance?