rust-core: add missing `Debug` traits
Turns out you need them when trying to use `hg-core` as a library. Who knew.
Differential Revision: https://phab.mercurial-scm.org/D8278
#require black $ cd $RUNTESTDIR/.. $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`