gershomb authored Dec 31, 2017 and Herbert Valerio Riedel committed Dec 31, 2017

`parserTrace label` is an impure function, implemented with "Debug.Trace" that
prints to the console the remaining parser state at the time it is invoked.
It is intended to be used for debugging parsers by inspecting their intermediate states.

These are simple yet helpful helpers for debugging Parsec parsers.

Closes #63