Reduce temporary objects during variable/filter parsing

Variable parsing uses a lot of temporary objects -- this change tries to reduce that somewhat by using =~ and Regexp.last_match instead of String.match.  

Benchmark of master (best of 3 runs):
```
                   user     system      total        real
parse:         2.070000   0.000000   2.070000 (  2.073355)
parse & run:   4.370000   0.000000   4.370000 (  4.384097)
```

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:         1.960000   0.010000   1.970000 (  1.973391)
parse & run:   4.290000   0.000000   4.290000 (  4.300754)

```

Object allocation profile for this PR:
```
==================================
  Mode: object(1)
  Samples: 8007400 (0.00% miss rate)
  GC: 0 (0.00%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
   2022100  (25.3%)     1299000  (16.2%)     Liquid::Variable#lax_parse
   9792000 (122.3%)      806100  (10.1%)     Liquid::Block#parse
    723100   (9.0%)      723100   (9.0%)     block in Liquid::Variable#lax_parse
    625200   (7.8%)      625200   (7.8%)     Liquid::Context#variable_parse
   2780700  (34.7%)      521600   (6.5%)     Liquid::Block#create_variable
    438600   (5.5%)      438600   (5.5%)     Liquid::Template#tokenize
    480300   (6.0%)      433300   (5.4%)     Liquid::Context#find_variable
   1716000  (21.4%)      421100   (5.3%)     Liquid::Context#resolve
   2521700  (31.5%)      300600   (3.8%)     Liquid::Variable#render
   1053600  (13.2%)      295500   (3.7%)     block in Liquid::Variable#render
    289100   (3.6%)      289100   (3.6%)     Liquid::If#lax_parse
   2259100  (28.2%)      238800   (3.0%)     block in Liquid::Block#create_variable
    204800   (2.6%)      204800   (2.6%)     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  (88.3%)      108700   (1.4%)     Liquid::Block#render_all
    600600   (7.5%)       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   (8.5%)       58100   (0.7%)     block in Liquid::Context#initialize
```

Down 163,800 objects and ~80ms.

@Shopify/liquid 

Otherwise, :+1: on this PR