Updates to FFI spec: performGC

[George Russell]

Alastair wrote about performGC (snipped)
> It'd be nice to say that it has to be a full GC - but I've no idea how
> to specify that in a non-operational (i.e., implementation dependent)
> way.
I certainly don't think you should constrain implementations to be able to perform
a "full GC" in any sense.  It is possible if unlikely that someone will get along
to implementing the "HaskellKit", where GC is entirely dispensed with and replaced by
region analysis.  Even if that's not the case, I certainly hope that one of these days
someone will get along to implementing a Haskell compiler that does at least some easy
region analysis.  Also there are probably hard-real-time GC algorithms (like Baker's treadmill) or
algorithms which are close to being hard-real-time (like the train algorithm) where doing a
"full GC" would be a major pain.