support async keyword at the stage of parse

Conflicts:

	src/classdef.jsx
	src/parser.jsx
add a failing test for newly introduced async keyword
add AsyncTask and Async classes to built-in.jsx
make keyword "async" contextual
add experimental async methods to nodejs compat module
add a simple todo test for async
disable async functions in nodejs library for the time being
clean up the async API
cleanup
fix a bug introduced in prev commit
[refactor] code transformer
add ExpressionTransformer class
add leaf expression transformers
[refactor] hide generator stuff from the compiler
add missing inheritances
[refactor] move utility methods to more specific transformers
[refactor] rename method
[refactor] unify types of members
[WIP] add some of expression transformers
small fixes