hi_sparse_bitset 0.9.0

Hierarchical sparse bitset. Incredibly high performance. Compact memory usage.
Documentation 
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# v.3

From v0.9.0

In little endian.
```text
version|lvl1_len|data_len||
lvl0_mask||
[lvl0_bitcount;8]||
[lvl1_mask;..]||
[lvl1_bitcount;..]||
[data;..]
```
Where
* `version` = `u16`
* `lvl1_len` = `u16`
* `data_len` = `u32`
* `lvl0_mask` = `Level0BitBlock`
* `lvl0_bitcount` = `Level0BlockIndices::Item`.
* `lvl1_mask`     = [`Level1BitBlock`; `lvl1_len`]
* `lvl1_bitcount` = [`Level1BlockIndices::Item`; `lvl1_len * size_of::<Level1BitBlock> / 8`]
* `data` = [`DataBitBlock`; `data_len`]
* `||` = padding to align next element.

For structure rationale see ImmutableBitset documentation for v0.9.0.