Multiply function could be 20% faster and smaller

All of the [constants](https://github.com/kokke/tiny-AES-c/blob/12e7744b4919e9d55de75b7ab566326a1c8e7a67/aes.c#L361-L364) needed by AES for the inversion of the MixColumns function fit in 4 bits, but both the function and macro versions of the `Multiply()` function support 5 bits. The size and performance of this could be improved by removing the case when [`((y >>4) & 1)` is set](https://github.com/kokke/tiny-AES-c/blob/12e7744b4919e9d55de75b7ab566326a1c8e7a67/aes.c#L326), since this can never happen with the constants used.

Start in aes.c at the Multiply() implementation and macro, along with the AES constants at lines 361-364. Confirm that the listed constants never set bit 4 of y. Done means both versions no longer handle that unreachable case while AES behavior remains unchanged.

Written by the indexing model from the issue text.