[WIP] Compressed posting lists

[xzfc]

This PR adds a draft implementation of the new sparse vector index format.

Comparison with the current implementation

Baseline: current dev branch, c173a9f.

Memory usage

	Old	New
Full data on-disk size	23.0G	15.4G
Index only (inverted_index.data) on-disk size	13.6G	5.9G
RAM consumption	13.9G	6.8G

Bench in sparse crate

New:

search/random_50k         time:   [1.0777 ms 1.0874 ms 1.0981 ms]
search/random_500k        time:   [8.9879 ms 9.1699 ms 9.3507 ms]
search/msmarco_1M         time:   [13.931 ms 14.254 ms 14.581 ms]
search/msmarco_full_0.25  time:   [32.163 ms 33.040 ms 33.934 ms]

Old:

search/random_50k         time:   [776.59 µs 781.35 µs 787.21 µs]
search/random_500k        time:   [7.2040 ms 7.3156 ms 7.4284 ms]
search/msmarco_1M         time:   [14.409 ms 14.760 ms 15.118 ms]
search/msmarco_full_0.25  time:   [33.352 ms 34.323 ms 35.312 ms]

MSMARCO

Results of https://github.com/qdrant/sparse-vectors-benchmark.
Times are in ms.

Full dataset

	Old	New
min	101.8	106.6
50p	248.9	228.6
95p	358.7	330.5
99p	403.8	382.2
999p	463.4	450.2
max	507.8	491.1

1M dataset

	Old	New
min	17.59	17.71
50p	36.24	31.63
95p	50.44	43.46
99p	58.26	50.56
999p	70.89	59.28
max	90.39	66.70