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.