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 martinus#94

…array

The previous patch allowed using the bucket array in segmented mode.

This patch expands the series to similarly also allow using a custom
container.

We are only allowing specifying a custom container and derive a possible
custom allocator directly by rebinding the value allocator. This
guarantees that they can be converted to each other.