Use a precomputed leaf marker in suffix trees

[gballet]

commitments for suffix trees are evaluated by breaking each 32-byte value into two 16-byte words, and then adding 1^128 in front of one of these two words, so as to distinguish a leaf with value 0 from a missing leaf.

This 1^128 can be precomputed during the polynomial evaluation, in order to reduce the time taken by the multi explonentiation.