Allow foreign import declarations to be split into type definition and "implementation" import

Instead of

foreign import ccall unsafe "lookup3.h _jenkins_hashword" hashWord
    :: Ptr Word32 -> CSize -> Word32 -> IO Word32

allow to write

hashWord :: Ptr Word32 -> CSize -> Word32 -> IO Word32
foreign import ccall unsafe "lookup3.h _jenkins_hashword" hashWord

Note: we can get close by

foreign import ccall unsafe "lookup3.h _jenkins_hashword"
 hashWord :: Ptr Word32 -> CSize -> Word32 -> IO Word32

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The motivation is that sometimes a function is natively implemented, sometimes it's a FFI import (depending on CPP, or through course of the project), and the vastly different syntax for function declaration is not convenient.