feat: add middleware check_witness

[ed255]

Complete the implementation of the check_witness function which checks that a witness assignment passes all the constraints from a middleware Circuit. The missing parts were:

• Simulate blind rows
• Check copy constraints
• Check lookups
• Check shuffles

I've tried to keep the implementation simple (compared to the MockProver). For now it panics instead of returning errors.

This function was previously defined in the p3_frontend but is now moved to halo2_debug so that it can be used anywhere.