Fixed String and Char inspect and dump methods. Also fixed String#to_json which now takes into a account control characters.

Added `\u` unicode escape sequences inside strings and chars (similar to Ruby). Added `String#chars`.

2048 fixes (close #184)

Added Tuple#clone and Tuple#dup. Faster tuple methods by using macros. Tuple#length and Tuple#[](index) (with a runtime index) are no longer primitives and are now implemented in Crystal.