Absyn post-processors now see the current grammar

Make monadsyntax more flexible with monad_unitbind

Previously, one had to bind "monad_unitbind" to a real constant, such as
option$OPTION_IGNORE_BIND. Now one can bind to a pattern, such as

\m1 m2. OPTION_BIND m1 (K m2)

This means that one doesn't have to define as many constants when
working with a fresh monad, which feels like an advantage.

Also clean-up monad selftest, and test new behaviour.

Move state_transformer stuff to its own directory src/monad.

This facilitates the addition of a FOR constant to state_transformerTheory.