benchmark commit step and ark counterparts

this MR adds a benchmark for

• the KZG10 trusted setup creating of ark-poly-commit
• the KZG10 commit of ark-poly-commit
• our own implement of the commit in zk::commit

there is also a slight improvement to the previous benchmarking of our zk::setup: the degree of the trusted setup is now computed once and for all before the benchmarking loop starts, because it's not what is of interest, let's not benchmark it.