* Make `unsafeInterleaveST` use `noDuplicate#` like
`unsafeInterleaveIO` does to prevent the suspended action from
being run in two threads.

* In order to accomplish this without `unsafeCoerce#`, generalize
the type of `noDuplicate#`.

* Add `unsafeDupableInterleaveST` to get the old behavior.

* Document unsafe `ST` functions and clean up some related
documentation.

Fixes #13457

Reviewers: austin, hvr, bgamari, ekmett

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3370