This is now https://isocpp.org/files/papers/P3764R0.html
"A utility function for propagating the most significant bit".

I've decided to drop everything except "sign_fill", which is now named
"msb_to_mask" for several reasons described in the proposal.

I'm not sure about widening bit-shifts yet; perhaps that could be made
part of Tiago's unified integer overflow arithmetic proposal, or maybe
it should be separate, but I kinda like this proposal being about just
one function.