This commit introduces ToDense and WriteDenseTo as methods in Bitmap.
They make conversion to dense bitmaps like https://github.com/bits-and-blooms/bitset
or https://github.com/kelindar/bitmap performant and simple.

```
name                                     time/op
WriteDenseTo/bitmap-4097-10                 135ns ± 2%
WriteDenseTo/run-4097-10                   40.2ns ± 0%
WriteDenseTo/array-4096-10                 8.74µs ± 0%
WriteDenseTo/bitmaps-and-runs-300000-10    2.29µs ± 0%

name                                     speed
WriteDenseTo/bitmap-4097-10              68.2GB/s ± 2%
WriteDenseTo/run-4097-10                 25.7GB/s ± 0%
WriteDenseTo/array-4096-10               1.05GB/s ± 0%
WriteDenseTo/bitmaps-and-runs-300000-10  24.6GB/s ± 0%

name                                     alloc/op
WriteDenseTo/bitmap-4097-10                 0.00B
WriteDenseTo/run-4097-10                    0.00B
WriteDenseTo/array-4096-10                  0.00B
WriteDenseTo/bitmaps-and-runs-300000-10     0.00B

name                                     allocs/op
WriteDenseTo/bitmap-4097-10                  0.00
WriteDenseTo/run-4097-10                     0.00
WriteDenseTo/array-4096-10                   0.00
WriteDenseTo/bitmaps-and-runs-300000-10      0.00
```

```
name                                     time/op
WriteDenseTo/bitmap-4097-10                17.0ns ± 0%
WriteDenseTo/run-4097-10                   29.4ns ± 0%
WriteDenseTo/array-4096-10                 8.65µs ± 0%
WriteDenseTo/bitmaps-and-runs-300000-10    1.52µs ± 0%

name                                     speed
WriteDenseTo/bitmap-4097-10               542GB/s ± 0%
WriteDenseTo/run-4097-10                 35.1GB/s ± 0%
WriteDenseTo/array-4096-10               1.07GB/s ± 0%
WriteDenseTo/bitmaps-and-runs-300000-10  36.9GB/s ± 0%

name                                     alloc/op
WriteDenseTo/bitmap-4097-10                 0.00B
WriteDenseTo/run-4097-10                    0.00B
WriteDenseTo/array-4096-10                  0.00B
WriteDenseTo/bitmaps-and-runs-300000-10     0.00B

name                                     allocs/op
WriteDenseTo/bitmap-4097-10                  0.00
WriteDenseTo/run-4097-10                     0.00
WriteDenseTo/array-4096-10                   0.00
WriteDenseTo/bitmaps-and-runs-300000-10      0.00
```

```
name \ time/op                        before         with-efficient-container  no-to-efficient-container
FromDense/bitmap-4097-10                1.45µs ± 1%               2.15µs ± 0%                1.47µs ± 2%
FromDense/run-4098-10                   1.04µs ± 4%               3.00µs ± 0%                1.04µs ± 2%
FromDense/array-4096-10                 1.45µs ± 1%               6.47µs ± 8%                5.46µs ± 1%
FromDense/bitmaps-and-runs-300000-10    3.53µs ± 0%              13.53µs ± 1%                3.57µs ± 1%

name \ speed                          before         with-efficient-container  no-to-efficient-container
FromDense/bitmap-4097-10              6.35GB/s ± 1%             4.28GB/s ± 0%              6.27GB/s ± 2%
FromDense/run-4098-10                  989MB/s ± 4%              345MB/s ± 0%               990MB/s ± 2%
FromDense/array-4096-10               6.36GB/s ± 1%             1.43GB/s ± 8%              1.69GB/s ± 1%
FromDense/bitmaps-and-runs-300000-10  15.9GB/s ± 0%              4.2GB/s ± 1%              15.8GB/s ± 1%

name \ alloc/op                       before         with-efficient-container  no-to-efficient-container
FromDense/bitmap-4097-10                8.22kB ± 0%               8.22kB ± 0%                8.22kB ± 0%
FromDense/run-4098-10                   8.22kB ± 0%               8.26kB ± 0%                8.22kB ± 0%
FromDense/array-4096-10                 8.22kB ± 0%              16.44kB ± 0%               16.41kB ± 0%
FromDense/bitmaps-and-runs-300000-10    8.35kB ± 0%               8.49kB ± 0%                8.35kB ± 0%

name \ allocs/op                      before         with-efficient-container  no-to-efficient-container
FromDense/bitmap-4097-10                  2.00 ± 0%                 2.00 ± 0%                  2.00 ± 0%
FromDense/run-4098-10                     2.00 ± 0%                 4.00 ± 0%                  2.00 ± 0%
FromDense/array-4096-10                   2.00 ± 0%                 4.00 ± 0%                  3.00 ± 0%
FromDense/bitmaps-and-runs-300000-10      6.00 ± 0%                16.00 ± 0%                  6.00 ± 0%
```