Pieter Wuille

@brunoerg Cool!

Rebased because of datedness.

Added a few commit messages to explain the rationale for the changes more.

> would there be some value in adding coverage to confirm that results aren't wildly off from what we have today if we use it more widely? Sure, happy to...

@instagibbs I incorporated a variant of your commit into the existing `feefrac_mul_div` fuzz test, with the tightest bounds I could make work.

> How can we be sure that this happens? A class has an implicitly-declared move constructor whenever: * There are no user-declared copy constructors. * There are no user-declared copy...

@maflcko `etd:is_nothrow_move_constructible_v` will also return true if a nothrow copy constructor is available (because a copy constructor is eligible in any context where a move constructor is called), so that...

@fanquake I've dropped the CI changes, as they're not really the main thing I want to achieve here. I do think there is value in reducing duplication of some groups...

> Nit: Add `build:` prefix to pull request title and remove "CI" mention from the pull request description? Done.

@ryanofsky I picked the name to match the same thing in [libsecp256k1](https://github.com/bitcoin-core/secp256k1/blob/v0.5.1/CMakePresets.json#L6), but I'm okay with "enable-all" or "all" as well.