Default out-marshall ForeignPtr

[EJahren]

It should be possible to give a FinalizerPtr to
a ForeignPtr hook so that it can be automatically
out-marshalled with

newForeignPtr finalizer

[ian-ross]

I've added some functionality to do this. You can now say:

{#pointer *test_struct as TestForeignPtr foreign finalizer free_struct#}

and the ForeignPtr will be created with the appropriate finalizer (and the correct foreign import declaration for the finalizer function will also be generated). Works for newtype ForeignPtrs too. The test case in tests/bugs/issue-73 shows how it works.