GC: optimize sweeping

[rainers]

Moves recovering small pages into sweep() and optimizes sweep by precalculating some common scenarios.
With the extended vdparser-test on win64:

• master: GC summary: 1355 MB, 13 GC 1443 ms, mark 1089 ms, sweep 199 ms, recover 150 ms, Pauses 1092 ms < 410 ms
• this PR: GC summary: 1355 MB, 13 GC 1226 ms, mark 1082 ms, sweep 16 ms, recover 124 ms, Pauses 1085 ms < 408 ms

[dlang-bot]

Thanks for your pull request, @rainers!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + druntime#2523"