segment_map: Also segment the bucket array

[StephanDollberg]

In segmented mode we only applied the segmenting to the values array but
not the bucket array.

As a result there the pattern of there still being a deallocation
followed by an increased allocation when resizing the hash map
continues to exist.

Further, in environments where the max allocation size is limited
because of fragmentation issues this can lead to problems.

To avoid both of these issues this patch makes the bucket array use the
same datastructure as the values array, i.e.: a std::vector when
linear and segmented_vector when segmented (or the passed
datastructure if specified).

This extra indirection does add some overhead in the segmented case.
Looking at the quick benchmarks we see:

Before:

|               ns/op |                op/s |    err% |          ins/op |          cyc/op |    IPC |         bra/op |   miss% |     total | benchmarking
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:-------------
|        8,912,995.09 |              112.20 |    0.1% |  225,712,537.08 |   26,628,198.00 |  8.476 |  25,133,812.23 |    0.1% |      1.15 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector iterate while adding then removing`
|       65,440,597.50 |               15.28 |    0.1% |  496,971,523.50 |  195,721,929.00 |  2.539 |  64,749,156.50 |   11.2% |      1.44 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector random insert erase`
|       63,254,162.50 |               15.81 |    0.1% |  540,753,642.50 |  188,790,381.00 |  2.864 | 101,168,500.00 |    6.3% |      1.39 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector 50% probability to find`
|        9,777,270.50 |              102.28 |    0.2% |  281,149,360.00 |   28,833,467.00 |  9.751 |  25,968,567.75 |    0.1% |      1.19 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector iterate while adding then removing`
|      220,368,952.00 |                4.54 |    0.2% |2,707,978,150.00 |  659,198,358.00 |  4.108 | 347,649,399.00 |    3.8% |      2.43 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector random insert erase`
|      156,887,435.00 |                6.37 |    0.1% |2,166,844,490.00 |  464,728,290.00 |  4.663 | 266,835,027.00 |    2.5% |      1.73 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector 50% probability to find`

After:

|               ns/op |                op/s |    err% |          ins/op |          cyc/op |    IPC |         bra/op |   miss% |     total | benchmarking
|--------------------:|--------------------:|--------:|----------------:|----------------:|-------:|---------------:|--------:|----------:|:-------------
|        8,921,748.31 |              112.09 |    0.1% |  226,313,644.69 |   26,684,106.00 |  8.481 |  25,174,702.92 |    0.1% |      1.18 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector iterate while adding then removing`
|       75,578,500.00 |               13.23 |    0.1% |  597,036,791.50 |  226,059,912.00 |  2.641 |  64,865,689.00 |   11.3% |      1.14 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector random insert erase`
|       74,928,542.00 |               13.35 |    0.1% |  677,557,943.00 |  223,726,152.00 |  3.029 |  91,606,575.00 |    7.0% |      1.13 | `ankerl::unordered_dense::map<uint64_t, size_t> segmented_vector 50% probability to find`
|       10,079,993.00 |               99.21 |    0.4% |  293,716,069.73 |   29,697,236.40 |  9.890 |  25,980,823.83 |    0.1% |      1.20 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector iterate while adding then removing`
|      220,081,085.00 |                4.54 |    0.1% |2,721,992,469.00 |  658,245,042.00 |  4.135 | 345,686,575.00 |    3.8% |      2.42 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector random insert erase`
|      158,126,693.00 |                6.32 |    0.1% |2,191,768,626.00 |  468,710,736.00 |  4.676 | 267,938,632.00 |    2.5% |      1.74 | `ankerl::unordered_dense::map<std::string, size_t> segmented_vector 50% probability to find`

If we think this is not unconditionally acceptable then we could
possibly add another template parameter (or make IsSegmented an enum) to
decide which parts are supposed to be segmented.

Fixes #94

[StephanDollberg]

@martinus I saw you reran the failed windows build but it failed again?

From what I can tell that issue is fmtlib/fmt#3540 which is supposed to be fixed in fmt 10.

[StephanDollberg]

Added another commit to also allow choosing a custom container for the bucket array.

This is also an option to allow only segmenting the values array.