## Summary
`BigDecimal.new` is deprecated since BigDecimal 1.3.3 for Ruby 2.5.
This PR suppresses the following warnings.
```console
% ruby -v
ruby 2.6.0dev (2018-09-06 trunk 64648) [x86_64-darwin17]
% RUBYOPT=-w bundle exec rake
(snip)
/Users/koic/src/github.com/Shopify/liquid/lib/liquid/utils.rb:49:
warning: BigDecimal.new is deprecated; use Kernel.BigDecimal method
instead.
/Users/koic/src/github.com/Shopify/liquid/lib/liquid/utils.rb:53:
warning: BigDecimal.new is deprecated; use Kernel.BigDecimal method
instead.
```
## Other Information
The following is a change of BigDecimal 1.3.3 for Ruby 2.5 related to this PR.
- https://github.com/ruby/bigdecimal/commit/533737338db915b00dc7168c3602e4b462b23503
- https://github.com/ruby/bigdecimal/commit/16738ad0aca158d3c70123990d8caa5789818177
Measures:
1) A while loop is faster than iterating with #each.
2) Check string, variable and block tokens first. They are far more
frequent than interrupt tokens. In their case, checking for an
interrupt can be avoided.
3) String tokens just map to themselves and don't need the special
treatment of BlockBody#render_node (except the resource limit
check).
Benchmark
=========
$ bundle exec rake benchmark:run
Before
------
Run 1)
parse: 41.630 (± 0.0%) i/s - 420.000 in 10.089309s
render: 75.962 (± 3.9%) i/s - 763.000 in 10.066823s
parse & render: 25.497 (± 0.0%) i/s - 256.000 in 10.040862s
Run 2)
parse: 42.130 (± 0.0%) i/s - 424.000 in 10.064738s
render: 77.003 (± 1.3%) i/s - 777.000 in 10.093524s
parse & render: 25.739 (± 0.0%) i/s - 258.000 in 10.024581s
Run 3)
parse: 41.976 (± 2.4%) i/s - 420.000 in 10.021406s
render: 76.184 (± 1.3%) i/s - 763.000 in 10.018104s
parse & render: 25.641 (± 0.0%) i/s - 258.000 in 10.062549s
After
-----
Run 1)
parse: 42.283 (± 0.0%) i/s - 424.000 in 10.028306s
render: 83.158 (± 2.4%) i/s - 832.000 in 10.009201s
parse & render: 26.417 (± 0.0%) i/s - 266.000 in 10.069718s
Run 2)
parse: 41.159 (± 4.9%) i/s - 412.000 in 10.031297s
render: 81.591 (± 3.7%) i/s - 816.000 in 10.018225s
parse & render: 25.924 (± 3.9%) i/s - 260.000 in 10.035653s
Run 3)
parse: 42.418 (± 2.4%) i/s - 424.000 in 10.003100s
render: 84.183 (± 2.4%) i/s - 847.000 in 10.069781s
parse & render: 26.726 (± 0.0%) i/s - 268.000 in 10.029857s
Justin Li