Improve the GC behaviour of IOArrays/STArrays

See Ticket #650

This is a small change to the way mutable arrays interact with the GC,
that can have a dramatic effect on performance, and make tricks with
unsafeThaw/unsafeFreeze redundant.  Data.HashTable should be faster
now (I haven't measured it yet).

We now have two mutable array closure types, MUT_ARR_PTRS_CLEAN and
MUT_ARR_PTRS_DIRTY.  Both are on the mutable list if the array is in
an old generation.  writeArray# sets the type to MUT_ARR_PTRS_DIRTY.
The garbage collector can set the type to MUT_ARR_PTRS_CLEAN if it
finds that no element of the array points into a younger generation
(discovering this required a small addition to evacuate(), but rough
tests indicate that it doesn't measurably affect performance).

NOTE: none of this affects unboxed arrays (IOUArray/STUArray), only
boxed arrays (IOArray/STArray).

We could go further and extend the DIRTY bit to be per-block rather
than for the whole array, but for now this is an easy improvement.