return Result([]const u8) instead of Result(void) from string parser

[jrachele]

Similarly to this issue: https://github.com/Hejsil/mecha/issues/42, the string parser returning Void on success is not congruent with the other parsers.

Now on success, the parser will return the string along with the rest, unless discarded via .discard()

[Hejsil]

Seems mecha did not compile with the latest zig version. I've fixed that, so can you rebase on master?

[Hejsil]

Thanks :+1:

[jrachele]

Cheers!