2670 - improvements to generics

Eliminated a few holes, gained more clarity on the shape of others.

Maybe I was sleep-deprived, but this was really hard until I wrote a few
unit tests directly on replace_type_ingredient.

Still one flaw remaining: the type-checker isn't smart enough to handle
'merge' for all the new cases. Tests pass since we don't use those
features outside C++ tests yet.

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