MINOR: ncbmbuf: define new ncbmbuf type

MINOR: ncbmbuf: define new ncbmbuf type

Define ncbmbuf which is an alternative non-contiguous buffer
implementation. "bm" abbreviation stands for bitmap, which reflects how
gaps and filled blocks are encoded. The main purpose of this
implementation is to get rid of the ncbuf limitation regarding the
minimal size for gaps between two blocks of data.

This commit adds the new module ncbmbuf. Along with it, some utility
functions such as ncbmb_make(), ncbmb_init() and ncbmb_is_empty() are
defined. Public API of ncbmbuf will be extended in the following
patches.

This patch is not considered a bug fix. However, it will be required to
fix issue encountered on QUIC CRYPTO frames parsing. Thus, it will be
necessary to backport the current patch prior to the fix to come.