Add the ability to statically define a `FunPtr` to a haskell function

Lauri Alanko suggests that there should be a way to define a callback to a haskell function statically (i.e. without using a dynamic wrapper).

It is currently possible to do so only by exporting the function and reimporting it as a function pointer:

foreign export ccall "my_callback" myCallback :: IO ()
foreign import ccall "&my_callback" myCallbackPtr :: FunPtr (IO ())

but of course this not ideal because it creates an extra symbol (and is quirky).

A possible new syntax could be:

foreign import ccall myCallbackPtr :: FunPtr (IO ()) = myCallback

or variations thereof. We probably want to keep import (rather than export or some new keyword), since this declaration brings something into scope, like a normal FFI import.

Trac metadata

Trac field	Value
Version	7.4.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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