ec/ecp_nistz256.c: switch to faster addition chain in scalar inversion.
ec/ecp_nistz256.c: switch to faster addition chain in scalar inversion.

[and improve formatting]

Reviewed-by: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5001)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl