Update to recent Rust nightly

As of rustc 0.13.0-nightly (7e11b2271 2014-12-24 20:47:12 +0000)
Rename: UTF8In -> Utf8Arg, IntoUTF8 -> IntoUtf8

This aligns with capitalization of std::str::Utf8Error and gets rid
of poorly readable "In".
Renamed from_static to from_static_str

As in my project c_compat.
gstr: Conversions which may fail return Result

This follows the changes in Rust libraries, works with try!() and
provides detailed error information.