* Rename bench_internal variables
The _x and _y suffices are confusing; they don't actually correspond
to X and Y coordinates. Instead replace them with arrays.
* Randomize the Z coordinates in bench_internal
Also increase the number of fe inputs.
* Make jacobi benchmarks vary inputs
Also make the num_jacobi benchmark use the scalar order as modulus,
instead of a random number.
* Add benchmark for secp256k1_ge_set_gej_var
This is a backport of secp256k1 PR797