MINOR: ncbmbuf: implement iterator bitmap utilities functions

MINOR: ncbmbuf: implement iterator bitmap utilities functions

Extend private API for ncbmbuf type by defining an iterator type for the
buffer bitmap handling. The purpose is to provide a simple method to
iterate over the bitmap one byte at a time, with a proper bitmask set to
hide irrelevant bits.

This internal type is unused for now, but will become useful when
implementing ncb_data() and ncb_advance() functions.

As the previous patch, this commit must be backported prior to the fix
to come on QUIC CRYPTO frames parsing.