Add 'deriving Show' for syntax & values for REPL

Also, remove duplicate `Show Term` instance in tests.
Update DEVELOPMENT.md for Show instance
Clarify DEVELOPMENT.md: GHCi is not needed & misc

On suggestion by Tillmann.
Allow getting module contents from REPL

This separates the phase which selects exported symbols among all
defined ones.

This also updates docs.
M ↦ Eval for clarity

Discussed with Tillmann. My main reason to fix this is for `:browse` in
`cabal repl`.
Autodetect literate files (fix #58)

Does not act on #55.
Syntax cleanup

I like Applicative syntax better.