pts.cabal: remove extra build-depends for pts exec

This effectively reverts 7185c76c7fd0cd5aa89188dccb24f151195da670, but
also removes other unneeded dependencies from pts.

cabal repl exe:pts won't work, but that's arguably a bug in cabal repl,
and as Tillmann pointed out, we don't really need that.
Create PTS.Interactive and DEVELOPMENT.md for REPL

Try to document and improve the REPL workflow, and add helpers for that.

For now, we just support easily creating terms.
Merge pull request #77 from Blaisorblade/topic/repl-ing

Better support for using the REPL