Deprecating STM invariant mechanism

[Ben Gamari]

tl;dr. Do you use Control.Monad.STM.always? If so say so on
       this [1] proposal otherwise the interface may be removed.


Hello everyone,

GHC's STM subsystem has long had the ability to run user-specified
invariant checks when committing transactions, embodied by the
Control.Monad.STM.always and alwaysSucceeds functions.

However, if Hackage is any indication this feature has seen very little
use of the past ten years. In fact, it has very likely been quite broken
(#14310) for this entire duration. 

Consequently, I suggest that we begin deprecating the mechanism. See
the deprecation Proposal [1] for full details. Please leave a comment if
you object.

Cheers,

- Ben


[1] https://github.com/ghc-proposals/ghc-proposals/pull/77
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