Fix memory leak in the VM assembler

[peterzhu2118]

The vm_assembler_element_t is not freed when the vm_assembler_pool_t is freed. This causes a memory leak to occur.

This script demonstrates the issue.

require "liquid"
require_relative "lib/liquid/c"

1_000_000.times do |i|
  Liquid::Template.parse("{{ value | default: 'None', allow_false: true }}")
end

puts `ps -o rss -p #{$$}`.chomp.split("\n").last.to_i

Before this patch: 124824

After this patch: 31116