MINOR: buffer: add a function to match against string patterns

MINOR: buffer: add a function to match against string patterns

In order to match known patterns in wrapping buffer, we'll introduce new
string manipulation functions for buffers. The new function b_isteq()
relies on an ist string for the pattern and compares it against any
location in the buffer relative to <p>. The second function bi_eat()
is specially designed to match input contents.