benchmark the recoding process

this MR

• adds criterion as a dependency
• creates a new benches/recoding.rs benchmark file
• makes the following public
  • fec::combine
  • field and field::split_data_into_field_elements

example results

bytes	shards	k	mean (us)
1	2	2	0.127
1	2	4	0.179
1	2	8	0.283
1	2	16	0.504
1	4	2	0.346
1	4	4	0.506
1	4	8	0.823
1	4	16	1.451
1	8	2	0.789
1	8	4	1.155
1	8	8	1.89
1	8	16	3.383
1	16	2	1.669
1	16	4	2.478
1	16	8	4.023
1	16	16	7.147
1024	2	2	1.02
1024	2	4	1.076
1024	2	8	1.172
1024	2	16	1.395
1024	4	2	2.981
1024	4	4	3.15
1024	4	8	3.453
1024	4	16	4.089
1024	8	2	6.907
1024	8	4	7.244
1024	8	8	7.969
1024	8	16	9.452
1024	16	2	15.169
1024	16	4	16.14
1024	16	8	17.086
1024	16	16	20.266
1048576	2	2	1470.966
1048576	2	4	1097.899
1048576	2	8	1091.298
1048576	2	16	1091.544
1048576	4	2	3274.852
1048576	4	4	3272.68
1048576	4	8	3251.877
1048576	4	16	3272.872
1048576	8	2	7582.074
1048576	8	4	7599.012
1048576	8	8	7584.59
1048576	8	16	7569.575
1048576	16	2	16274.986
1048576	16	4	16303.905
1048576	16	8	16313.429
1048576	16	16	16310.305