Support for char & string literals

[leissa]

Added support for char and string literals.

• 'x' is a literal of type I8.
• "ABC\0", is a tuple (`A`, `B`, `C`, `\0`) of type «4; I8».
• Note that Thorin doesn't care about null-termination or whatever. So in order to use null-terminated strings, you have to take care of \0 by yourself.
• Check out lit/str.thorin as a test case/example.
• As 'x doesn't work anymore as a rebind operator, this branch changes this to `x instead.
• Rn, this only adds ASCII support with the usual escape chars from C.

We probably want to use this for #210.