This introduces a new `__scope_guard` without any fancy features. The
scope guard is used in `<string>` to simplify some of the ASan
annotations (especially by making it harder to forget them where
exceptions are thrown).
`<string>` doesn't seem to be required at all and `flat_map` doesn't
support `vector<bool>`, so we can include just `vector<T>`. This cuts
the include time in half on my system.
Around half of the tests are based on the tests Arthur O'Dwyer's
original implementation of std::flat_map, with modifications and
removals.
partially implement #105190
