Tidy source tree

  - Separate assignments and unifications in ANF.  Assignments are now
    exclusively for loading (or checking) literals.
  - Move term base cases out from our myriad of representations into
    Dyna.Term.TTerm's TBase.
  - Fix comma and whenever handling in ANF (which is a temporary hack)
    and while there, make "true" and "false" a little closer to correct.
  - Move Analysis.Aggregation over to Main.Exception rather than Either.
  - Remove some dead code from Backend.Python and ParserHS.Parser
  - While here and there, squash some warnings