parse test helper, and test solution, too
Inverse shim for PuzzleRead->PuzzleParse conversion.

This isn't quite perfect, but will allow writing tests
against the current implementation that don't need
changes after conversion.
Add tightfit parsing tests.
Show instance for Grid s a (yikes!)
more Show
remove redundant RawPuzzle constructor
More tests that test more.

Tests weren't failing earlier, since the Yaml parsing succeeded,
and the errors were hidden due to missing evaluation. Hack around
this by Show'ing the parsed value.