pytd optimizations, AST tweaking.
pytd optimization: factorizing, optional arguments
Add optimization: merge types into common base type.
Better class lookup. Add "decorate" helper. ShortenUnions optimization.
Add pyttypedecl/builtins/

Add standard library (sys, posix, etc.) and builtin types as pytd.
Add new function utils.GetBuiltins().

Allow to store modules in an AST.
Remove builtin functions from builtins/__builtin__.pytd that have
a type with the same name.