use Net::SSLeay (OpenSSL) for SHA-(1|256) if installed

use Net::SSLeay (OpenSSL) for SHA-(1|256) if installed

On my x86-64 machine, OpenSSL SHA-256 is nearly twice as fast as
the Digest::SHA implementation from Perl, most likely due to an
optimized assembly implementation.  SHA-1 is a few percent
faster, too.