Generators are now building AST
Type aliases are now using `quote_item!`
gen_enum_item now returns an ast::Item
to_rust_ty now returns a ast::Ty
Turn more code in global_gen into AST generating code
Now using AST for parameters and return type generation
Partially ASTify write_failing_fns
Add some documentation
Merge pull request #173 from tomaka/cleanup-3

Clean up implementation (step 3)