Request for review of proposed strict-concurrency improvements

[Yitzchak Gale]

During the past few days, David Feuer has proposed a number of
improvements to the strict-concurrency library. Thanks very much for
this work!

That, in turn, was triggered by some ideas of Eric Crockett - again,
thanks very much for this!

Since there are some subtle nuances at play here, I am asking the
community to give this a brief review. Please comment directly in the
PRs or issues, links below, not here on the list. Absent any comments,
I will merge these PRs in about a week. If it's needed sooner than
that, please let me know.

Thanks,
Yitz

PRs:
https://github.com/ygale/strict-concurrency/pull/9
https://github.com/ygale/strict-concurrency/pull/10

Issues:
https://github.com/ygale/strict-concurrency/issues/2
https://github.com/ygale/strict-concurrency/issues/3
https://github.com/ygale/strict-concurrency/issues/4
https://github.com/ygale/strict-concurrency/issues/5
https://github.com/ygale/strict-concurrency/issues/6
https://github.com/ygale/strict-concurrency/issues/7
https://github.com/ygale/strict-concurrency/issues/8