ecc_mod_inv interface and allocation changes.

ChangeLog

curve25519-eh-to-x.c

curve25519-mul.c

ecc-ecdsa-sign.c

ecc-ecdsa-verify.c

ecc-eh-to-a.c

ecc-internal.h

ecc-j-to-a.c

ecc-mod-inv.c

examples/ecc-benchmark.c

testsuite/ecc-modinv-test.c