This introduces a minor change to the structure of MCode Let
bindings. Before various refactoring, a Let would use a CombIx to
indicate where execution should resume once the binding is
evaluated. Now the code is resolved ahead of time and referred to
directly. However, logically this code is just a further Section;
the rest of the actual function that contained the Let.

The purpose of using a Comb was to store stack protection
information, so that the stack check can be performed upon reentry.
This is necessary if the continuation has been captured and we are
resuming in a context that was not the original entry point of the
resumption.

Now, though, a top level 'comb' can be either code or a memoized
value, but a resumption will never be the latter. So Let has been
changed to store the relevant information, instead of delegating to
Comb.

…into cp/cache-toplevel

A PAp should only contain an actual combinator, not a cached value.
So, the combinator case has been factored out of and unpacked into
GComb. This way a PAp can refer to the factored-out part.

This allows code sent between machines to pre-evaluate things if
applicable. Naturally, this requires serialization version changes.
Some tweaks have been made to avoid changing hashes as much as
possible between versions.

Old serialized values and code can still be loaded, but obviously
they will be treated as completely uncacheable.

…into cp/cache-toplevel

It was meant to be a test in a `match` expression, but was missing
a #:when