* Split 'consts' collection by content types into:
- literals: for literal expressions, like `"a"`
- classes: for character class expressions, like `[a]`
- expectations: for constants describing expected values when parse failed
- functions: for constants with function code
* Move any JavaScript code generation from 'generateBytecode' to 'generateJs'.
* Rename opcode 'MATCH_REGEXP' to 'MATCH_CLASS' (name reflects purpose, not implementation).
* Replace 'PUSH' opcode with 'PUSH_EMPTY_STRING' opcode because it is only used with empty strings
Before this commit, the PEG.js parser always created the AST using a plain JavaSctript object, but allthough simple and effective for the job, this method sacrificies performance slightly.
From now on the parser shall call a Node creator. This should help with performance, as well as in the future move some AST helpers into the new AST functions.
