feat: variable block size & elias-fano encoding

[silver-ymz]

bench

• machine: 8 vCPU, 32 GB, AMD Ryzen 7 7700, hyper-v Virtualization

dataset: trec-covid

before the PR:
Index: 4.5458s (37689.88/s)
Query: top1000 - 15.9687s (31.31/s), top10 - 18.0994s (138.13/s)
Index disk size: 384 MB

fixed block partition + delta bitpack:
Index: 4.7356s (36179.51/s)
Query: top1000 - 12.9719s (38.54/s), top10 - 13.9670s (178.99/s)
Index disk size: 508 MB

fixed block partition + elias fano:
Index: 4.8131s (35597.16/s)
Query: top1000 - 13.1613s (37.99/s), top10 - 13.3764s (186.90/s)
Index disk size: 508 MB

variable block partition + delta bitpack:
Index: 9.2832s (18456.21/s)
Query: top1000 - 13.1917s (37.90/s), top10 - 13.4416s (185.99/s)
Index disk size: 508 MB

variable block partition + elias fano:
Index: 9.3289s (18365.81/s)
Query: top1000 - 12.9883s (38.50/s), top10 - 12.4730s (200.43/s)
Index disk size: 513 MB

dataset: arguana

before the PR:
Index: 2.1008s (4128.82/s)
Query: top1000 - 47.9059s (29.35/s), top10 - 35.7517s (39.33/s)
Index disk size: 260 MB

fixed block partition + delta bitpack:
Index: 2.5946s (3343.05/s)
Query: top1000 - 36.7708s (38.24/s), top10 - 19.0610s (73.76/s)
Index disk size: 347 MB

fixed block partition + elias fano:
Index: 2.6600s (3260.88/s)
Query: top1000 - 36.0452s (39.01/s), top10 - 18.4784s (76.09/s)
Index disk size: 347 MB

variable block partition + delta bitpack:
Index: 2.7843s (3115.37/s)
Query: top1000 - 33.9380s (41.43/s), top10 - 18.7480s (74.99/s)
Index disk size: 347 MB

variable block partition + elias fano:
Index: 2.7671s (3134.68/s)
Query: top1000 - 33.4785s (42.00/s), top10 - 17.4946s (80.37/s)
Index disk size: 347 MB