Fix benchmarks & add IPA proving benchmark

This PR:

Fixes benchmarks since cargo bench was broken due to compilation errors, probably related to API changes.
Added an extra IPA proving benchmark.
Fixed all lints in benchmarks.
Refactored/unified some logic a bit to avoid repetition.

Results in my machine (AMD Ryzen 7 3800XT):

ipa - prove (256)  
     time:   [42.831 ms 42.904 ms 42.983 ms]                            
ipa - verify (multi exp2 256)
     time:   [6.2486 ms 6.2682 ms 6.2938 ms]
ipa - verify (256)
     time:   [24.013 ms 24.052 ms 24.098 ms]                               
multipoint - verify (256)/1
     time:   [6.9494 ms 7.0065 ms 7.0834 ms]
multipoint - verify (256)/1000
     time:   [23.630 ms 23.695 ms 23.773 ms]
multipoint - verify (256)/2000
     time:   [39.499 ms 39.556 ms 39.625 ms]
multipoint - verify (256)/4000
     time:   [68.280 ms 68.457 ms 68.685 ms]
multipoint - verify (256)/8000
     time:   [117.84 ms 118.03 ms 118.27 ms]
multipoint - verify (256)/16000
     time:   [212.41 ms 212.98 ms 213.57 ms]
multipoint - verify (256)/128000
     time:   [1.4578 s 1.4617 s 1.4657 s]
multiproof - prove (256)/1
     time:   [54.056 ms 54.252 ms 54.500 ms]
multiproof - prove (256)/1000
     time:   [67.173 ms 67.239 ms 67.311 ms]
multiproof - prove (256)/2000
     time:   [79.433 ms 79.772 ms 80.157 ms]
multiproof - prove (256)/4000
     time:   [93.425 ms 93.769 ms 94.160 ms]
multiproof - prove (256)/8000
     time:   [131.29 ms 131.85 ms 132.43 ms]
multiproof - prove (256)/16000
     time:   [204.26 ms 205.18 ms 206.17 ms]
multiproof - prove (256)/128000
     time:   [1.2434 s 1.2472 s 1.2509 s]

cc @kevaundray

crate-crypto / ipa_multipoint

Fix benchmarks & add IPA proving benchmark #11