Compressed posting lists

[xzfc]

#4143

This PR adds compressed posting list implementation to the sparse crate.

• New module: index::compressed_posting_list, akin to index::posting_list.
• A new trait PostingListIter. Both of the above implement it.
• New modules: index/inverted_index/inverted_index_compressed_{immutable_ram,mmap} (akin to index/inverted_index/inverted_index_*).
• Updated benchmarks. Now they compare four implementations (old ram/mmap; new ram_c/mmap_c).
• Integration is not included.

Benchmark results

Times in milliseconds.

basic

dataset	ram	ram_c	diff	mmap	mmap_c	diff
random-50k	0.87684	1.1668	+33%	0.87264	1.2039	+37%
random-500k	8.0528	10.766	+33%	8.0214	10.691	+33%
neurips2023-1M	13.085	17.295	+32%	13.093	17.613	+34%
neurips2023-full-25pct	30.481	40.589	+33%	30.720	40.745	+32%
movies	4.0141	6.0397	+50%	4.1774	6.0223	+44%

hottest

dataset	ram	ram_c	diff	mmap	mmap_c	diff
random-50k	0.18766	0.21201	+12%	0.1876	0.21118	+12%
random-500k	1.6244	1.8405	+13%	1.6245	1.8403	+13%
neurips2023-1M	9.1040	10.591	+16%	9.1037	10.582	+16%
neurips2023-full-25pct	20.094	23.968	+19%	20.190	23.856	+18%
movies	0.20035	0.26813	+33%	0.20207	0.26778	+32%