[crypto] parallelize witness commitment and interpolation

[mimoo]

did some experiments following discussion with @imeckler :

use par_iter to parallelize commitment of witness columns (and interpolation)

some bench

without par_iter in the codebase:

Proof creation/proof creation (SRS size 2^11)                        
                        time:   [481.88 ms 483.59 ms 485.32 ms]

Proof creation/proof creation (SRS size 2^15)                        
                        time:   [6.2940 s 6.3487 s 6.4120 s]

with par_iter (current master):

Proof creation/proof creation (SRS size 2^11)                        
                        time:   [409.67 ms 413.74 ms 417.99 ms]
Benchmarking Proof creation/proof creation (SRS size 2^15): Warming up for 3.0000 s

Proof creation/proof creation (SRS size 2^15)                        
                        time:   [3.2189 s 3.3084 s 3.4077 s]

with par_iter for witnesses (this PR):

Proof creation/proof creation (SRS size 2^11)                        
                        time:   [398.51 ms 410.01 ms 424.46 ms]

Proof creation/proof creation (SRS size 2^15)                        
                        time:   [3.1958 s 3.2787 s 3.3728 s]

not the greatest improvement, perhaps someone will see something I didn't

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