DAL: test `share_is_trap`

This MR tests the Trap.share_is_trap function:

• checks that the number of traps is as expected over many shards
• checks the edge cases: traps_fraction is 0 or 1
• benches the running time

For the running time, the DAL benchmark from tezt/tests/dal.ml is updated. Here is the data, see last line:

SRS ............................................................................ 1         1182.488ms 100%
(shards: 512, slot size: 126944, redundancy_factor: 8, page size: 3967) ........ 1         3379.977ms 100%
  make ......................................................................... 1            0.005ms 200%
  shard precomputation ......................................................... 1         2064.298ms 100%
  slot generation .............................................................. 1            1.327ms 101%
  polynomial from slot ......................................................... 1            1.667ms 100%
  commit ....................................................................... 1           52.522ms 100%
  prove commitment ............................................................. 1          204.983ms 100%
  shards from polynomial ....................................................... 1           10.415ms 100%
  prove shards ................................................................. 1          838.172ms 100%
  Reconstruct polynomial ....................................................... 1           40.822ms 100%
  verify commitment ............................................................ 1            0.822ms 100%
  verify shard (size: 2052) .................................................... 1            2.959ms 100%
  verify shard multi (size: 2052) (number_of_shards:1) ......................... 1            3.013ms 100%
  verify shard multi (size: 2052) (number_of_shards:5) ......................... 1            3.700ms 101%
  verify shard multi (size: 2052) (number_of_shards:20) ........................ 1            5.396ms 100%
  verify shard multi (size: 2052) (number_of_shards:100) ....................... 1           12.773ms 100%
  verify shard multi (size: 2052) (number_of_shards:512) ....................... 1           42.020ms 100%
  prove page ................................................................... 1           45.774ms 100%
  verify page .................................................................. 1            4.036ms 100%
  share_is_trap (number_of_shards:512) ......................................... 1           43.482ms 100%