editor: make for overlay, highlight editable vals.

fix test suite

Add form shown by <show> tags that allow you to update values

Add basic ability to update value via json api

Add binary serialization & a redis source backend.

Look up source values when evaling

eval takes source lookup fn, returns IO.

Eliminate (pointless) Var wrapper

Add another test.

Add <let> tag.

tc returns type env, evalTemplate takes envs.

Add some tests for template rendering

lams get return types, so can tc recursive ones

add == prim

rename ParseE to Grammar, <eval> to <show>