Unify BN_rshift design
Unify BN_rshift design

This commit aims at refactoring the `BN_rshift` by making it a wrapper
around `bn_rshift_fixed_top`, in order to match the current design of
`BN_lshift`, as suggested in the discussion at
https://github.com/openssl/openssl/pull/10122#discussion_r332474277 .

As described in the code, by refactoring this function, `BN_rshift`
provides a constant-time behavior for sufficiently[!] zero-padded inputs
under the following assumptions: `|n < BN_BITS2|` or `|n / BN_BITS2|`
being non-secret.

Notice that `BN_rshift` returns a canonical representation of the
BIGNUM, if a `fixed_top` representation is required, the caller should
call `bn_rshift_fixed_top` instead.

Reviewed-by: Nicola Tuveri <5a38b4ac03c90be61ead22c1d11b6e9f49425984@gmail.com>
Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10196)

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