Plonk : Split T − Fixes #116

Change T’s split.

The number of parts is now decided by deg(T), subgroup_size and SRS’s size. All parts (except the last one) have the size of the subgroup, and their number is equal to deg(T)+1 // subroup_size.

When r = deg(T)+1 mod subroup_size is not zero, the last part will be of size subgroup_size + r if the SRS is long enough to handle it ; else there will be one more part of subgroup’s size & the last part will be of size r.

If zero_knowledge is false SRS size is set up to n, else n+9 (n = subgroup size).