Define fe_invert() in terms of invsqrt()

Define fe_invert() in terms of invsqrt()

Pros:
- The code is a couple lines shorter.
- Stack usage may be lowered a tiny little bit (we saved two temporary
  field elements).

Cons:
- Inversion is one field multiplication slower.

Note that fully optimised inversion can be 6 field multiplications
faster than the current version. It would cost quite a bit of code
though (we'd need a dedicated addition chain).