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]`.

test/unit/optimizer_test.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class OptimizerUnitTest < Minitest::Test
+  include Liquid
+
+  def test_combines_append_filters
+    optimizer = Optimizer.new
+    var = Variable.new('hello | append: "a" | append: b', ParseContext.new)
+    var = optimizer.optimize(var)
+    assert_equal([
+      ['append_all', ["a", VariableLookup.new("b")]]
+    ], var.filters)
+  end
+end