Introduce some functions to act on tuples.

This commit introduces fst/1, snd/1, curry/1 and uncurry/1. The first two
functions are similar to erlang:element/2 and the later two are functions that
return functions to do with tuples.

More general forms of curry/uncurry are possible but are not here implemented.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>