Added support for -bf-byte-counts

Byfl can now describe working-set sizes in terms of the number of
bytes needed (e.g., of cache) to cover various fractions of the total
number of memory accesses.  The goal is to replace -bf-reuse-dist with
a substantially faster alternative.

This revision also includes some code refactoring.  All library-common
definitions and external declarations of functions and variables have
been moved into byfl.h.