Add a method OpenSSL::PKey#size

I was trying to migrate an old Perl script to Ruby, which receives a string consisting of a signature and the contents concatenated. The core of the script is like this:

my $sign_key = Crypt::OpenSSL::RSA->new_public_key($key_string);
my $signature = substr($data, 0, $sign_key->size, ''); # Cut off signature of $data
die "data wasn't signed correctly!\n" unless $sign_key->verify($data, $signature);

There does not appear a way to get the size of the signature from Ruby. It should just be a call to EVP_PKEY_get_size

Start at the implementation of the OpenSSL::PKey API and trace how EVP_PKEY-backed methods are exposed to Ruby; the issue identifies EVP_PKEY_get_size as the underlying entry point. Done means OpenSSL::PKey#size reports the key or signature size needed by the concatenated-data use case, with behavior checked against the OpenSSL documentation.