Better markdown benchmarks

[Kerollmops]

This PR displays the best result in bold and gives you the size of the temporary files after all benchmarks, so you can get a rough idea of their total size. I also make sure the table is in Markdown format, making it easier to copy and paste into the README.

	redb	lmdb	rocksdb	sled	sanakirja
bulk load	3.90s	1.25s	7.07s	5.57s	1.13s
individual writes	459.81ms	8.09ms	4.34ms	406.12ms	57.15ms
batch writes	4.78s	2.72s	272.26ms	1.41s	4.70s
len()	4.42µs	416.00ns	207.18ms	722.56ms	46.73ms
random reads	882.28ms	943.44ms	2.59s	1.15s	830.85ms
random reads	886.02ms	980.67ms	2.51s	1.14s	843.01ms
random range reads	2.01s	1.38s	4.65s	4.62s	1.21s
random range reads	2.37s	1.36s	4.74s	5.82s	1.24s
random reads (4 threads)	322.40ms	251.22ms	815.59ms	372.30ms	335.81ms
random reads (8 threads)	239.00ms	178.85ms	619.15ms	312.32ms	512.79ms
random reads (16 threads)	271.99ms	155.67ms	550.53ms	304.05ms	622.27ms
random reads (32 threads)	268.41ms	159.45ms	511.69ms	330.67ms	673.09ms
removals	3.60s	1.17s	3.03s	1.91s	1.16s
size after bench	1.00 GiB	584.85 MiB	383.33 MiB	453.00 MiB	4.00 GiB

Those benchmarks were done on my Macbook Pro M1.

[cberner]

Looks like an improvement. Thanks! Can you fix the clippy errors and I'll merge it?

[Kerollmops]

I just rebased, highlighted the best results of README, and fixed the Clippy warnings.

[cberner]

Merged. Thanks!