Move untested code out of SDK

verify_signature() was only used in a test that was testing
binary layout. It only worked because the test transaction only
had one signature.

from() was only used by verify_signature() and that's something
we'd typically called `pubkey()`.

hash() didn't return the hash of the Transaction, as you might
guess. It's only used for PoH, so move it into Entry.

core/src/sigverify.rs

@@ -392,7 +392,6 @@ mod tests {
             Some(sig_start as usize)
         );
         assert_eq!(sig_len, 1);
-        assert!(tx.verify_signature());
     }
 
     #[test]