constify *_dup() and *i2d_*() and related functions as far as possible,...

constify *_dup() and *i2d_*() and related functions as far as possible, introducing DECLARE_ASN1_DUP_FUNCTION

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8029)