More strictly check bounds in FACT trampolines

[alexcrichton]

Note: this is temporarily rebased on #13026

This commit is a hardening of the various in-bounds checks and such of
the FACT compiler, in particular as related to strings. The previous
implementation would check bounds in a few places but this was a bit
ad-hoc and not uniformly done. There's no known issue with the prior
checks, but given the sensitive nature of these checks I feel it's best
to make this a bit more rigorous.

Specifically the malloc helpers, and a newly added realloc helper,
will internally verify not only alignment but additionally the size of
the allocation itself. All manual invocations of realloc are switched
over to this helper. Additionally all conversion of a guest pointer to a
more structured value now additionally goes through helpers which
performs these same checks to ensure that everything is in-bounds.

The net result is that this should have no behavior change from before.
A suite of tests are added for behavior around large strings,
specifically exercising the maximum allowable size of strings. This
uncovered a few minor issues in transcoding where spec-wise Wasmtime
previously transcoded too many bytes before performing a
growing realloc.

Finally a few refactorings were done in FACT to handle some helpers
going away, notably around translating the map<K, V> type, which
cleans up the internals as well.