New functions ecc_mod_mul_canonical and ecc_mod_sqr_canonical.

New functions ecc_mod_mul_canonical and ecc_mod_sqr_canonical.

* ecc-mod-arith.c (ecc_mod_mul_canonical, ecc_mod_sqr_canonical):
New functions.
* ecc-internal.h: Declare and document new functions.
* curve448-eh-to-x.c (curve448_eh_to_x): Use ecc_mod_sqr_canonical.
* curve25519-eh-to-x.c (curve25519_eh_to_x): Use ecc_mod_mul_canonical.
* ecc-eh-to-a.c (ecc_eh_to_a): Likewise.
* ecc-j-to-a.c (ecc_j_to_a): Likewise.
* ecc-mul-m.c (ecc_mul_m): Likewise.

(cherry picked from commit 2bf497ba4d6acc6f352bca015837fad33008565c)