[CI test, dontmerge] PR #1000 with __(u)mul128() based abstractions in int128_struct

[sipa]

[roconnor-blockstream]

It may be a good idea to output a preprocessor warning in this case, or actually remove the simulated 64x64->128 mul code because our tests will never exercise it.)

We could add a #warning pragma saying "Beware of bugs in the above code; We have only proved it correct, not tested it."

[sipa]

Superseded by #1156.