Add OPENSSL_hexstr2buf_ex() and OPENSSL_buf2hexstr_ex()

Add OPENSSL_hexstr2buf_ex() and OPENSSL_buf2hexstr_ex()

They do the same thing as OPENSSL_hexstr2buf() and OPENSSL_buf2hexstr(),
except they take a result buffer from the caller.

We take the opportunity to break out the documentation of the hex to /
from buffer conversion routines from the OPENSSL_malloc() file to its
own file.  These routines aren't memory allocation routines per se.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9303)