Add enable_monadsyntax as alias for add_monadsyntax; document it all

Closes #523

Complete implementation of new API for declaring+using monads

Monadsyntax in general (the do..od notation) can be turned on and off.
In addition, specific monad instances can be declared and subsequently
enabled by just calling something like

monadsyntax.enable_monad "option"

Having state_transformerTheory as an ancestor turns on monadsyntax, and
enables the state monad. (Enabling a monad causes a bunch of calls to
overload to occur; disabling a monad undoes those calls.)

Provide machinery for declaring/exporting monad information

Declare options, lists, state-transformers (bad name!) and
errorStateMonads as monads. For the moment, this infrastructure is not
used, but next step will be to link it into the parsing and printing
machinery.

Change monadsyntax to NOT change global grammar on load

Instead, users must explicitly call {temp_,}add_monadsyntax to turn do
.. od and the <- syntax on, along with the pretty-printing and
augmented parsing.

Tidy monad-syntax printing tests

* Make sure that printing is not tested with two printers simultaneously
installed
* Check printing of "_unitbind" form

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.

extra semicolons removed from (some) .sig files

Move state_transformer stuff to its own directory src/monad.

This facilitates the addition of a FOR constant to state_transformerTheory.