Move Pair from Prim to Lambda
Dup & Jam --> dup & jam.

Replaced `Dup` and `Jam` by `dup` and `jam`, defined as `id &&& id`
and `id ||| id`, respectively. Pretty-print back to `dup` and `jam`.
swapP, swapC; examples
example tweak
tidying
Simplify and ShowFolded.