refactor: num limbs

[brozorec]

There are already another two draft PRs tackling the exact same problem #365 and #367

Here, I propose a slightly different approach that has the following benefits:

1. Less refactoring of the existing code: NUM_LIMBS is also passed as a generic but with a default value.
2. No need to pass another generic when initializing Limbs: NUM_WITNESS_LIMBS is removed; this value is a function of NUM_LIMBS, so it can be computed. Added a new derive macro FieldsCols that computes this value and constructs FieldDenCols, FieldOpCols and FieldInnerProductCols structs.

With this PR, I'd just like to propose a rough outline of what can be done and see if this approach passes the team's temp check. Ready to collaborate with @0xkanekiken and eventually merge our works.

[puma314]

Closing in favor of this approach: #417