Give names/descriptions to a couple of tests in monad selftest

Unit tests for monadic let syntax

Fix more regression tests broken by testutils API change

Fix error in identifying bound variables in monad actions

Closes #657

Fix another conditional-expression p/printing oddity

The if-then- block wasn't necessarily aligned with the last else block
because the whole if-then-else wasn't guaranteed to appear in one
block to itself.

Closes #588

Also tweak error-reporting from testutils.tpp (use ␣ (U+2423) for
spaces to make spotting differences easier; control colours better).

Add (commented-out) test for monad+if pretty-printing failure

This is effectively the test from github issue #588

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.