WIP for change to list syntax handling.

Very broken at the moment

Use grammar-deltas for user-provided absyn post-processors

Preserve add_user_printer properly in grammar data

This requires a slightly involved process for storing references to
code in theory files.

Update the .doc file on add_user_printer (which was already incredibly
long), and add a test.

Closes #367

Handle clear_overloads_on with user-deltas

Make 'remove-rules-with-tok' a user-delta

Handle add_numeral_form et al with user-deltas

Also take the opportunity to remove redundant add_actual_overloading
entrypoint in Overload module.

Add deltas for moving overload operations

Sadly, making an inferior overload is not *quite* the same as making a
normal one, then then moving it "to the back", so I've left both
operations in the user_delta type for the moment.

Handle gen_remove_ovl_mapping

This gets tests as far as string_numTheory

Begin process to use GrammarDeltas inside theories

This doesn't work yet (just attempting to load boolTheory leads to an
exception), but it's close to working, at least for some situations. I'm
generating new constructors for the user_delta type more or less as the
errors prompt me to.

The new constructors in this commit are IOVERLOAD_ON for "add inferior
overload on" (it's not really clear that this is used enough to warrant
inclusion, but hey); ASSOC_RESTR for "associate restriction", which is
used in boolTheory and probably nowhere else ever, and RMOVMAP for
"remove overload mapping", which is used to remove overloadings to
constants.

General pattern in Parse is to generate the user_delta values, and to
then feed these to the term_grammar.add_delta function to make the
grammar updates. This makes Parse simpler on the whole, which is
definitely an improvement.

Record overload_on calls in GrammarDeltas

Add 2 rule-removal constructors to user_delta type

These will help with concretisation of grammar changes in theory files.

Remove BRULE constructor from user_delta type

It is redundant, though the GRULE representation of a grammar's binder
rule contains information that is never used.

Properly re-org grammar code, removing rule_fixity

Now there is just one fixity type. Also decide that set_fixity and
set_mapped_fixity shouldn't be in the grammar-delta type; instead the
"remove-termtok" and "add_rule" actions (which can be used to implement
the above) should be.

Refactor more grammar code

Ultimate is to switch to concrete "grammar-deltas" in theory files
rather than calls to the update_grms function. Start this process by
extending existing delta type with SET_MAPPED_FIXITY constructor. This
is not yet used.

Refactor grammar datatypes into separate struct

This removes duplicated datatype declarations