some minor adjustments mean i can get more cores going round trip now, but still failing on some huge cases.
Merge pull request #3 from gseitz/master

Use equalA and context bounds for Equal.
added small, failing test case so i can fix it later.
some initial progress on Shrink[Core[V]]
removed the failing core, because ive distilled the problem down to floats and doubles. all other cores work.
Add Show instance for Core.

Still keeping (Var|Data|App).toString for consistency with
the other Core data constructors that are case classes.