Turn Pos into a newtype

Preparation for trying to speed Text ops back up.
Text: allow ensure to tell us how far to advance
Switch Text to Word16-based indexing

This seems to work (!!), and rescues us from the massive slowdown
of the exceedingly dumb first implementation.
Move number parsing benchmarks into their own module

--HG--
rename : benchmarks/Benchmarks.hs => benchmarks/Numbers.hs