More accurate allocation stats for :set +s
The point of this commit is to make the memory allocation statistic from :set +s in GHCi a lot more accurate. Currently it uses the total allocation figure calculated by the RTS, which is only updated during GC, so can be wrong by an arbitrary amount. The fix is to the the per-thread allocation counter that was introduced for allocation limits. This required changes to the GHC API, because we now have to return the allocation value from each evaluation. Rather than just change the API, I introduced a new API and deprecated the old one. The new one is simpler and more extensible, so hopefully we won't need to make this transition in the future. See GHC.hs for details.
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