Refactor

[epost]

fyi @wisnesky I'm shuffling some things around a bit.

• [x] Add containers dependency.
• [x] Move typeclass constraints out of data Term definition.
• [x] Divide stuff up into lib / test / app code.
• [x] Make code compile; move non-compiling stuff to tests where possible.
• [x] Align stuff, fix indentation, spacing, etc.
• [x] Replace stringly typed stuff by ADTs.
  • [x] typeOf
  • [x] various Either String error types