GeometryCollection: Cache flags to avoid O(n) lookups

[dbaston]

As discussed in locationtech/jts#1100. For a collection with 1 million elements, reduces CoverageUnion runtime by 98%.

Increases the size of collections from 88 to 96 bytes. This could be avoided by storing the flags in Geometry (which has 4 spare bytes) rather than GeometryCollection (which has none).

This PR:

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Benchmark                                        Time             CPU   Iterations
----------------------------------------------------------------------------------
BM_CoverageUnion<SegmentSet>/1000          1309915 ns      1309904 ns          544
BM_CoverageUnion<SegmentSet>/4096          5467171 ns      5466976 ns          124
BM_CoverageUnion<SegmentSet>/32768        54011812 ns     54009415 ns           12
BM_CoverageUnion<SegmentSet>/262144      498335728 ns    498323707 ns            2
BM_CoverageUnion<SegmentSet>/1000000    2130948900 ns   2130788554 ns            1
BM_CoverageUnion<BoundaryChain>/1000        866139 ns       866082 ns          745
BM_CoverageUnion<BoundaryChain>/4096       3223064 ns      3222934 ns          216
BM_CoverageUnion<BoundaryChain>/32768     27829346 ns     27825710 ns           25
BM_CoverageUnion<BoundaryChain>/262144   222791662 ns    222777788 ns            3
BM_CoverageUnion<BoundaryChain>/1000000  976485195 ns    976432682 ns            1

main:

----------------------------------------------------------------------------------
Benchmark                                        Time             CPU   Iterations
----------------------------------------------------------------------------------
BM_CoverageUnion<SegmentSet>/1000          1204288 ns      1204243 ns          587
BM_CoverageUnion<SegmentSet>/4096          5243907 ns      5243915 ns          130
BM_CoverageUnion<SegmentSet>/32768        51261928 ns     51258943 ns           13
BM_CoverageUnion<SegmentSet>/262144      477953902 ns    477906954 ns            2
BM_CoverageUnion<SegmentSet>/1000000    2052768394 ns   2052653441 ns            1
BM_CoverageUnion<BoundaryChain>/1000        989152 ns       989121 ns          723
BM_CoverageUnion<BoundaryChain>/4096       5400333 ns      5399829 ns          123
BM_CoverageUnion<BoundaryChain>/32768    109018236 ns    109002983 ns            6
BM_CoverageUnion<BoundaryChain>/262144  5086413577 ns   5085701992 ns            1
BM_CoverageUnion<BoundaryChain>/1000000 3.7859e+10 ns   3.7757e+10 ns            1

[dr-jts]

Increases the size of collections from 88 to 96 bytes.

Seems like a low-percentage increase.