initial "foreign import prim" patches for review

[Isaac Dupree]

Simon Marlow wrote:
> foreign import prim "has_side_effects commutable foo#" :: ...

note, FFI spec defines lack-of-"safe/unsafe" to mean "safe" (and it's 
*not* unsafe in the typical FFI ways, so I think that makes a bit more 
sense to me than "unsafe")

If there are features like "can_fail" and "has_side_effects" whose 
absence leads to an optimization, it makes me a little uncomfortable 
(what if they're not specified when they should be? didn't we have a bug 
like that for has_side_effects just recently, even with the primops all 
specified in one well-commented file?)  Also would it be better to call 
it "ghcprim" rather than "prim" (then people will be more wary of all 
these compiler-specific requirements), or does it not really matter?

-Isaac