Support performing prefix queries using UTF-8 byte string

DAWG currently has methods like `b_has_key()` and `b_get_value()`, but no `b_prefixes()`. Also, `prefixes()` method accept only unicode string.

I use DAWG for extracting pre-defined dictionary words from arbitrary UTF-8 texts. Since converting long key into UTF-8 at every call can significantly degrade the performance, I think it would be very nice to support `b_prefixes()` or a similar method that enables to perform prefix queries using UTF-8 byte string.