Optimize uint256 multiplication

[chfast]

1. We want to use the "loop" procedure as in mul_loop_opt().
2. There, it may be good to separate first iteration which does not need to load p[] values as they are zeros. Compilers are not always able to figure it out.
3. The best score was 60 instructions.
4. Some experimentation: https://godbolt.org/z/915aeG.

[chfast]

New Instructions Supporting Large Integer Arithmetic

[chfast]

https://godbolt.org/z/GhfKv31vT

[chfast]

Prototype of u192 multiplication with mulx,adox,adcx: https://godbolt.org/z/6nbc3aafE