Again: FFI syntax

[Simon Marlow]

>    * Silly combinations like "an unsafe label" or 
>      "a dynamic import with a given C-name" don't exist.

But now we have all new silly combinations, like

	foreign export "dynexp" foo :: ...
and
	foreign export "dynimp" foo :: ....
and
	foreign export "&foo" foo :: ...

the extent field needs to be separated into import-extent and
export-extent, I think.

Can we make the "static" optional?  It wouldn't introduce any ambiguity,
and it would save characters in the common case.

I like '&', but I'm less sure about '!' - this feels like we're getting
a little too cryptic.

Cheers,
	Simon