Add utf8/utf16 constructors for RelativeTo

[Manishearth]

Currently it must be constructed from a str, it would be nice for FFI if we had from_utf8 and from_utf16 like the others.

[nekevss]

This should be possible once the new release of ixdtf and we've bumped the version.

[Manishearth]

I might preemptively add those ctors to ffi, then, so we can at least start using it with conversion in the bridge code.

[Manishearth]

Done: #375