[phobos] More GC stuff

[David Simcha]

I'm looking a little more at low-hanging fruit to improve GC performance,
and I think I may have found yet another huge performance bug, but I want to
make sure I'm not horribly misunderstanding the code before I try to "fix"
it.  This one has nothing to do with the patch I already submitted.

Someone please take a look the loop on lines 2479-2500 of gcx.d (
https://github.com/D-Programming-Language/druntime/blob/master/src/gc/gcx.d).
It looks to me like B_PAGEPLUS blocks of memory are *getting scanned O(N^2)
times, completely by accident.  *As far as I can tell, the variable u
represents the number of pages in the block currently being examined.  o
represents some offset from the base of the pool.  Therefore, it looks like
we're scanning the same memory multiple times for absolutely no reason.  I
also wonder if stuff that's not supposed to be getting scanned is.  o can be
greater than the base of the block, but we're marking from o to o + u *
PAGESIZE, where u * PAGESIZE is the length of the whole block.
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