Add a fast 16x16=16 multiplication algorithm

[colinleroy]

Hope I did things correctly :)

16 bit x 16 bit unsigned multiply, 16 bit result (low-bytes)
Average cycles: 138.35
2144 bytes

[colinleroy]

Thanks :)
By the way, would you be interested in setting up a division_test repo? I found it hard to find resources on fast division for the 6502. In the project where I need division (16 bits by 8 bits), I used an approximate algorithm based on inverse multiplication and that 16x16=16 multiplication algorithm: https://github.com/colinleroy/a2tools/blob/master/src/lib/approxdiv16x8.s
I'd love to see other options (ideally faster 😅)!