Feature req: Support for Extension Fields

[einar-inversed3]

We would like to compute over extension fields, such as Goldilocks/x^2+7 or Babybear/x^4+11.

Before we start, we wanted to ask you if this is something you would be interested in implementing upstream, collaborating about, receiving a PR for or rather not?

[dot-asm]

Babybear extension in question is implemented as bb31_4_t. As for
Goldilocks please contact hello@supranational.net to discuss.