Optimizer should remove equal-constrained witness columns.

[chriseth]

The optimizer seems to not remove equal-constrained witness columns.

When run on the poseidon_gl std test:

cargo run   pil -f test_data/std/poseidon_gl_test.asm 

The optimized pil contains the following code:

col witness X11_const;
main::X11 = main::X11_const;

Both X11 and X11_const are witness column.

I would propose the following:
If we have a polynomial constraint of the form w = v, where both w and v are witness columns, then replace all occurrences of w by v (replace the longer witness columns with the shorter one) and remove w.

Note that we cannot do that for array elements currently.