Renaming hirestime to spanclock. As that is a fairer description. It's a tool
for measuring time spans as accurately as possible. Which is not the same thing
as providing a running clock with a very high fidelity.
updated api to reflect hirestime -> spanclock, anoyingly looks like
I should have commited the rename separately as spanclock.h is listed as a new
file rather than descendent of hirestime.h
* Made windows deal with QueryPerformanceFrequency internaly.
* removed spanclock_freq (windows was the only platform it was used on)