Performance improvements (#15)

* Performance improvements

Two main changes:

1. Cache parsed AST in a Source. So Parser will parse content only once.
2. Use one unified visitor with multiple callbacks instead of multiple
visitors to traverse AST.

This improves performance significantly, for example running it on
Crystal repository takes ~1 second, which 6 times faster than before.

* Change readme example

spec/ameba/ast/traverse_spec.cr

@@ -8,7 +8,7 @@ module Ameba::AST
     {% for name in NODE_VISITORS %}
       describe "{{name}}" do
         it "allow to visit {{name}} node" do
-          visitor = {{name}}Visitor.new rule, source
+          visitor = Visitor.new rule, source
           nodes = Crystal::Parser.new("").parse
           nodes.accept visitor
         end