Tried to use libm for calculating logarithmic frequencies of words,

Tried to use libm for calculating logarithmic frequencies of words,
but ran into problems with function name collisions. Damn the C's flat
namespace!

Renamed our log to log_msg, but keep the original name as a macro
expanding to the new one. Also renamed log2 (which is currently not used
anywhere) to fls (find last set, akin to ffs).

Introduced lib/math.h which is a wrapper around <math.h> handling
name collisions by clever macro tricks.