ReadWriteMemory::get_polys_r1cs by reference

[sragss]

ReadWriteMemory::get_polys_r1cs() is extremely slow (here) because it clones all of the underlying not once but twice. On a ~16M cycle count trace on a 64 core machine it takes 2% of end-to-end time.

First: remove the double clones from self.v_read and self.v_write_rd. Double clones come from a clone in DensePolynomial::evals() then another in .flat_map(...).collect().

Second: refactor get_polys_r1cs() to return lifetimed references to the underlying vectors via DensePolynomial::evals_ref() rather than cloning / reallocating at all.

It may be helpful to review the tracing strategy for performance testing.

[MatteoMer]

hey! Would be happy to take this!

[sragss]

Great! Feel free to ask questions here.

[moodlezoup]

Closed by #307