Make types generic over encoding

[Manishearth]

See #109

Currently we have BidiInfo and then utf16::BidiInfo types (with other similar duplication).

It would be nice if instead we had BidiInfo<T>, ParagraphInfo<T>, etc, which all used some trait input like TextInfo; rather than having duplicate types for everything. We might need some kind of UTF8 UTF16 marker type for this.

I don't have the time to do this myself but it would be really neat. Especially if we can extend stuff to UTF32 as well as WTF8 and other ill-formed UTF8.