Make ecc modular inversion use redc form, for relevant curves.

Make ecc modular inversion use redc form, for relevant curves.

* ecc-mod-inv.c (ecc_mod_inv_destructive): New helper function,
not preserving input argument. Extracted from old ecc_mod_inv.
(ecc_mod_inv): Call ecc_mod_inv_destructive.
(ecc_mod_inv_redc): New inversion function, with input and output
in redc form.

* ecc-secp224r1.c: Select between ecc_mod_inv and ecc_mod_inv_redc.
* ecc-secp256r1.c: Likewise.

* ecc-j-to-a.c (ecc_j_to_a): Simplify redc-related logic, taking
advantage of ecc->p.invert handling redc, when appropriate. Reduce
scratch need from 5n to 4n in the process (assuming inversion
needs 2n).

* testsuite/ecc-modinv-test.c (ref_modinv): Updated to do redc, if
appropriate.