Replace IO manager's IntMap with a mutable hash table

I've written a patch that replaces the immutable !IntMap used by GHC.Event with a mutable hashtable, !IntTable.

There's a standalone version of the new data structure, complete with !QuickCheck tests and benchmarks, available on github. It's about 15x faster than !IntMap, and substantially simpler.

In practice, this translates to a small but measurable improvement in throughput (and presumably latency). I see a 3% to 10% bump in requests handled per second by the tiny acme-http http server when benchmarked using the weighttp load tester.

Trac metadata

Trac field	Value
Version	7.7
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
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CC	AndreasVoellmy, kazu-yamamoto, simonmar
Operating system
Architecture