Crate stable-borrow-state regressed in 1.28 beta

[pietroalbini]

The test suite of the stable-borrow-state crate failed to run, and after a quick glance at the source code I think that's caused by a change in the RefCell behavior.

• stable-borrow-state regressed from stable to beta (build log) cc @tcr

[tcr]

This crate was intentionally used as a stopgap until I could rely on a nightly feature, and months later is fully unused. I'm going to yank it.

[Mark-Simulacrum]

Regression caused by aec00f9 - #51466.

This is unsurprising as the library was transmuting to get at internal details. Closing.