Deal with rebindings of ptree_{size,fringe}_def in grammarScript

Ability to EVAL these constants is lost because of b98f5ead5

Not sure that anybody ever used this, but still. With regression
test.

First steps towards parsing f ⦇ d1 ↦ r1; d2 ↦ r2 ⦈ for fn. UPDATE

The ASCII version of the same is f (| d1 |-> r1; d2 |-> r2 |) and of
course, the old (d1 =+ r1) ((d2 =+ r2) f) can also be used.

The pick of ⦇ ... ⦈ precludes using the same parentheses for
s-expressions in the example of the same name (hence the changes in
that example). The use of (| |) means that (||) no longer parses as
paren-escaping the || operator (hence the change to wordsScript.sml).

Pretty-printing still to be done.

Modify testutils.require_msg and derivatives to return values

It can be useful to get back the result of a passing test for use in
further tests that build on that.

Knock-on effects where it is now not possible to simply List.app such
functions; they must now have their results explicitly ignored.

Have testutils.die automatically print out "FAILED"

Argument to die is then appended to that, but on a new line.

Adjust some selftests to take advantage of this, though of course, we
don't expect to see the FAILED output for any of them because they're
all passing.

Add pretty-printing test-cases for s-expression terms

These cases currently fail, but shouldn't.

Cleaner approach of location in parser

Fixed selftest

Make context-free languages selftest use testutils

One more parsing test for repeated "dot-conses"

Improve sexp parsing to handle dotted lists better

Also move some peg_eval theorems from CakeML to pegTheory and use these
to simplify proof about valid symbols.

I think a symbol should not start with a . or a "

It would probably be possible to allow symbols starting with ".", but is
it worth the effort?

Also, I noticed a discrepancy with my understanding of sexps:
(this is a dotted . list) is not allowed, but I think it should be
(representing (this . (is . (a . (dotted . list)))))

make pegLib.derive_lookup_ths return more

playground for formal language theory developments.