fanquake

Note that the last version from upstream was nearly 4 years ago: https://github.com/arun11299/cpp-subprocess/releases/tag/v2.0.

> Maybe a subtree from our own fork? I think that would mostly just re-introduce the awkwardness we ended up with with univalue, for not much benefit, and for code...

Concept ACK - I think moving to using the builtins is ok. With the eventual plan to migrate to `std::byteswap`. @aureleoules everytime I visit the [coverage/benchmarks for this PR](https://corecheck.dev/bitcoin/bitcoin/pulls/29263), I...

@theuni want to rebase this now that #29404 is in?

Looks like the test coverage report now no-longer shows any significant difference in benchmarks.

> Drop from 26.0 milestone? Why?

> make the option to disable them a bit easier to find. Not sure. If we aren't completely convinced about actually dropping them yet, I'm not sure that making the...

Closing for now. This is a simple change, but doesn't make sense yet, and can easily be reopened / redone once there is a clear path forward for #25725.

There are more PRs open. i.e #29428.

> Maintainer note: NO_BRANCH=true must be set in Cirrus for bitcoin-core/gui before merging this. See https://cirrus-ci.com/github/bitcoin-core/gui -> Settings. Can you rename this to something that actually explains what it's doing;...