change lib omp link lib name + fixes to stdlib

[vezenovm]

Previously nargo would not even build in the CI. I changed the link for non ARM based linux to use omp5 rather than omp. I held off on changing the linker for ARM_LINUX but we should probably test this and change for ARM_LINUX build as well before merging. It looks that reason nargo was failing in the CI is that ubuntu-latest requires the omp5 flag rather than omp. In order to build on my local machine I needed the omp flag. We have changed the CI build to use ubuntu-20.04 as in this PR (noir-lang/noir#565), so I switched the libomp link to use omp. We can make the change to discern between ubuntu versions once we plan to upgrade the CI to use ubuntu-latest. For now, I think keeping the link name as omp is safest.

This PR also makes a couple fixes to std_lib funcs and DSL glue that was causing issues with Noir.

1. How we constructing the merkle_membership constraints was incorrect as we were treating the hash path as a flattened list of pairs, however, we switched this in Noir to have a singular hash path as the merkle inclusion proof. I made a change in the C++ to reflect this.
2. schnorr had an unnecessary assert to check whether the points provided are on the grumpkin curve. This assert was causing failures when trying to fetch the circuit size without witness data. I replaced this assert with constraints to check whether a point is on the curve.