Revised FFI syntax + typing

[Simon Marlow]

> -- allowed for import/export
> callconv : 'ccall'                   -- default
>          | 'wincall'
>          | 'stdcall'                 -- deprecated, same as wincall
>          | 'cplusplus'
>          | 'jvm'
>          | 'dotnet'

A totally minor point, but 'wincall' doesn't feel right.  This
alternative calling convention has been around since long before windows
(it's always been the default calling convention for Pascal, I think).

I'd stick with 'stdcall' because that's what everyone else seems to call
it.  gcc has a 'stdcall' function attribute, BTW.

> word_ty          : 'Word8'
>                  | 'Word16'
>                  | 'Word32'
>                  | 'Word64'
>                  | 'Word'            -- GHC extension

Why is Word a GHC extension?  Someone remind me?

Cheers,
	SImon