Improve non-native arithmetic gadget

[ivokub]

Non-native arithmetic gadget has now been implemented. Collecting small snippets and improvement ideas for later improvement:

• [x] feat: add Field structure which has methods which return emulated.Element instead of frontend.Variable. Then don't have to cast to emulated.Element when explicitly needed (for example for sw_emulated point coords). This would also provide type safety between Field and Element.
• [x] perf: implement fast paths for cases where inputs are constants
• [x] refactor: we differentiate between constant and variable. Add checks to ensure user does not try to use constant as value receiver (they still get panic, but for other reasons and isn't helpful for the user)
• [x] refactor: check that the elements belong to the same field in operations.
• [x] refactor: add sanity checks before the operations (e.g. that overflow is sufficient and do not need to reduce)
• [x] refactor: split variable.go into variable.go and params.go
• [x] perf: for lookup2 and select we need that the inputs have the same number of limbs and overflow. But actually the can be different, just the result has to have maximum number of limbs and overflow
• [x] feat: remove Placeholder when compiling circuits.
• [x] perf: maybe there is a more efficient method for checking that multiple bits are zeros.
• [x] bug: #348
• [ ] perf: try to have more efficient splitting of a variable (instead of doing full binary decomposition and recomposition of the slices, create only the needed split)
• [ ] perf: consider the case where the emulated field is a lot smaller than the native field. If the operations never overflow the native field, then maybe there are more efficient ways for reducing the values (we would be interested in Goldilocks field).
• [ ] feat/perf: theoretically it is possible to optimise modular exponentiation using Wesolowsky RSA VDF verification. Not high-priority yet.
• [ ] feat: implement Sqrt, Double, Half, Square
• [ ] perf: look into Montgomery form for representing elements. It may allow for more efficient modular reduction after multiplication (a la Aztec/Jellyfish)
• [ ] perf: when initialising constant from big.Int, then instead of creating all nbLimbs limbs, create only sufficient number of limbs to fit the constant. NB! in this case we should be certain that the placeholder limb count corresponds to the witness limb count.
• [ ] test: implement tests for expected failures
• [ ] related: #416 - add methods on Field type which also take the Element receiver to modify in-place.
• [ ] feat: implement parametrised std/selector.Mux and std/selector.Map. Right now the selectors work only on slices of variables. However, maybe we could parametrise the methods so that we could use the functions on emulated.Element and other more complex types.
• [ ] use #547 for internal hints.

[ivokub]

The tasks are nice-to-have future developments for non-native arithmetic. Proposing to postpone for the next release after including more efficient range checks.