REMOVED pragma

Say I have a function that's been deprecated for some time. Then I remove it. Some clients will still use the now-missing function. It would be nice to give a helpful error message, instead of a "symbol not found" error.

I thus propose a REMOVED pragma. The syntax is identical to DEPRECATED, except for the name of the pragma. Whenever a REMOVED function is mentioned, the error message includes the string from the pragma. (This includes trying to define a REMOVED class method in an instance.)

See https://gitlab.haskell.org/ghc/ghc/-/wikis/design/deprecation-mechanisms for more info and related efforts.

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Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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