Use a GitHub action to run test on each push and pull request.
We test across a matrix covering Linux and MacOSX, and Go 1.8 to 1.15.

A previous attempt to add non-determinism to the diffing algorithm
unfortunately broke the algorithm for half the cases.

This change modifies the algorithm to truly switch between starting
with a forward search versus a reverse search.
The main for-loop of Difference would switch repeatedly between
performing a forward search, then a reverse search, and vice-versa.
Since we can't jump into the middle of a for-loop to start with the
reverse search first, we use a series of labels and goto statements
to accomplish the same effect.

Fixes #238

Map keys should have a sensible verbosity limit imposed,
otherwise the reporter can end up printing a massive data structure
that cannot reasonably fit in memory.