Use Go strings instead of C.CString

[qmuntal]

We can define all C strings passed from Go to C as good all Go constant strings casted on demand using unsafe.StringData, which is almost no-op.

My main motivation here is to eliminate all C.CString calls, as they have some small performance penalty (2 cgo calls, one for C.CString itself and the other for the companion C.free) and it is easy to forget calling C.free after.

A nice side-effect is that now string global variables can be statically initialized by the linker thanks to them being constant strings. This will result in faster initialization times.