Proposal: Make unsafeInterleaveST safer

[David Feuer]

Currently, unsafeInterleaveST is considerably less safe than
unsafeInterleaveIO. Specifically, very careful discipline is required
to make code using it thread-safe. This means that unsafeInterleaveST
actually behaves like unsafeDupableInterleaveIO. See Edward Kmett's
blog post from several years ago [*] for details.

I propose that we should

1. Make unsafeInterleaveST behave only as unsafely as unsafeInterleaveIO, and

2. Add a new unsafeDupableInterleaveST function implementing the
current unsafeInterleaveST behavior.

The alternative would be to leave unsafeInterleaveST alone and add a
new unsafeNonDupableInterleaveST, but that seems like an absolutely
terrible mismatch of names. Since any currently-correct code using
unsafeInterleaveST will continue to work (albeit more slowly), I don't
think that's justified.

Thanks,
David

[*] https://www.schoolofhaskell.com/school/to-infinity-and-beyond/older-but-still-interesting/deamortized-st#newsflash--unsafeinterleavest-is-unsafe-
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