`assign_advice` copy instead of return reference

[jonathanpwang]

For direct comparison, this branch https://github.com/axiom-crypto/halo2/tree/axiom/advice-without-lifetimes does assign_advice where the value is copied and returned, versus in branch axiom/faster-witness-generation where only a reference of the value is returned.

halo2-ecc bench msm takes ~650ms witness generation in the former (without lifetimes), while times are more like ~550ms in the latter.

This change is a sub-change of #2 so it explains a portion of the time difference there.