Gadget migration to bellpepper-gadgets

[huitseeker]

Umbrella issue:

This internal Nova repo uses a ton of gadgets, mostly extracted from https://github.com/alex-ozdemir/bellman-bignat

Some PRs, such as #273 end up redefining a ton of gadgets themselves, since the idiosyncratic ones contained in the repo are sometimes not the most useful.

We have a more general repository in https://github.com/lurk-lab/bellpepper-gadgets that :

• contrarily to this repo, should host most reusable gadgets, and provide ample room to test them,
• contains bellpeppe-emulated, a high-quality crate for emulated field arithmetic, which should be used rather than ad-hoc solutions.

Further, bellpepper defined the core gadgets (bool, num ..) and should host their utilities:

• https://github.com/lurk-lab/bellpepper

Daughter issues

See also argumentcomputer/bellpepper-gadgets#43 (which is this issue on the bellpepper-gadgets side) and #324 which is a specialization to ECC.

Prior PRs (examples)

• Import Conditional selection from Nova bellpepper#85
• Refactor to use bellpepper's new conditional selects #355