feat: implement non-native field emulation

[ivokub]

This PR adds a new standard gadget std/math/nonnative for performing non-native field operations in a circuit. It is trying to follow big.Int-like interface, but is not completely compatible.

The PR is mostly ready, but still some things to do:

• add tests for more complex computations
• implement Select and Lookup2
• write a nice documentation which lists all the assumptions the library does to ensure the validness of the computation

[ivokub]

Marking this version as ready to review as is. It is functionally complete, but lacks several optimisations I will implement in a new PR. When reviewing, I suggest to start from std/math/nonnative/doc.go (or look at the package documentation with godoc) for general description and then (in parallel) look at std/math/nonnative/variable.go.

[SherLzp]

will it support Sqrt? @ivokub

[ivokub]

will it support Sqrt? @ivokub

Currently Sqrt is not implemented in the PR, but it should be easy to add. Will add the Sqrt method soon.

[SherLzp]

when will this PR be merged? @ivokub

[ivokub]

when will this PR be merged? @ivokub

I believe it is being reviewed right now. Cannot promise ETA but hopefully shortly.