Add generic type parameters to CodeMap and File

[Tazdevil971]

Hi! I needed to store something other than a String inside CodeMap but currently that's not supported.

The idea would be to add a generic type T to both CodeMap and File, this type would need to implement AsRef<str>, then add a getter method to codemap::File that would allow you to access this underlying type in an unmutable way, or even change codemap::File::source directly.