Introduce the AST Optimizer

... that can only optimize one thing: combine `| append: 'a' | append: 'b'` into a single `| append_all: ['a', 'b']` filter call.

This avoids all the intermediate allocations caused by several `append`.

Also introduce a new filter `append_all: [*items]`.

lib/liquid.rb

@@ -84,6 +84,7 @@ require 'liquid/usage'
 require 'liquid/register'
 require 'liquid/static_registers'
 require 'liquid/template_factory'
+require 'liquid/optimizer'
 
 # Load all the tags of the standard library
 #