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This is the mixins package, so "Mixin" in filenames is redundant.

This diff is pretty fun.  It's our first alternative representation,
and it all works out nicely (no internal syntactical complications
encountered or any other unexpected unpleasantness).

It's also our first representation that involves potentially recursive
destructuring work over a scalar!  So, you can see the new 'construct'
method conventions appearing to handle that.  Works out neatly.
This kind of recursive destructuring happens all within the span of
processing one "token" so to speak, so naturally it can work without
any stateful machinery.

Some utility functions are added to the mixins package.
(It's sort of betraying in the name of the package, but, well,
it's an extremely expedient choice.  See comments about import
management.  tl;dr: consistency makes things easier.)

Tests for a recursive case of this will be coming soon, but requires
addressing some other design flaws with the AdjunctConfig maps.
(StructField is... not a good key.  It's not always hashable... such
as when the StructField.Type field is inhabited by TypeStruct.)
Easy enough to fix, just not going to cram it into this commit.

The way null and the 'fcb' are handled here differs from previous
generators: here, we *always* have an 'fcb', and just make a special
one for the root builder.  This is... well, it works.  It was an
attempt to simplify things and generate fewer method overrides, and
it did.  However, I'm not going to dwell on this too long, because...

The 'fcb' system is not working out well overall for other reasons:
it's causing costly allocations.  (Taking a function pointer from
a method requires at least two words: one for the function pointer
and one for the value to apply it on.  That means an allocation.)
So a serious rewrite of anything involing the 'fcb' strategy is needed.

And that is going to be a little tricky.  The 'fcb' idea was itself
a trying-slightly-too-hard-to-be-clever attempt to avoid an alternative
solution that involves generating additional types per distinct child
value slot (and I'm still unwilling to pursue that one -- it suggests
far too many types).  The next best idea I have now seems to involve a
lot of pointers into other assembler's state machines; this will surely
be a bit touchy.  But no worse than any of the rest of all this, I
suppose: it's *all* "a bit touchy".  Buckle up for fun diffs ahead.)

So, considering these 'fcb' notes, this feels a bit mixed...
"Two steps forward, one step back", so to speak.  Still: progress!
And it's very exciting that we got through several new categories of
behavior without hitting any other new roadbumps.