Implement raw tag parsing in BlockBody

[peterzhu2118]

See Shopify/liquid#1378.

Benchmarks

In a microbenchmark for raw tags, this PR introduces a performance impact during parsing.

This branch:

               parse     80.202k (± 8.7%) i/s -    400.816k in   5.036587s
              render    214.678k (± 6.1%) i/s -      1.081M in   5.055158s
      parse & render     48.202k (± 5.8%) i/s -    243.744k in   5.074713s

Master:

               parse    105.637k (± 6.9%) i/s -    535.392k in   5.092843s
              render    206.248k (± 8.5%) i/s -      1.036M in   5.058648s
      parse & render     57.954k (± 6.6%) i/s -    292.464k in   5.069258s

Script:

require "benchmark/ips"
require "liquid"
require "liquid/c"

source = <<~LIQUID
  {% raw %}
    {% test %}
    hello world
    {% endtest %}
  {% endraw %}
LIQUID

template = Liquid::Template.parse(source)

Benchmark.ips do |x|
  x.report("parse") do |times|
    times.times do
      Liquid::Template.parse(source)
    end
  end

  x.report("render") do |times|
    times.times do
      template.render!
    end
  end

  x.report("parse & render") do |times|
    times.times do
      Liquid::Template.parse(source).render!
    end
  end
end

[peterzhu2118]

Closing since #135 is merged.