stabletailgraph: implement stable-tail sort This adds the computation of the "stable-tail sort", an incremental node sorting method. It is a stepping stone for the implementation of faster label discovery (for example for obs markers) and more caching.

heptapod: add `.gitattributes` file to improve language detection I am fully aware of the irony.

peer: rename makepeer() → _make_peer() In httppeer and sshpeer, there previously were makepeer() and make_peer(), which was confusing. Therefore, this changeset renames one of the functions. makepeer() was the internal function called by make_peer() and some debug command. This function is renamed to _make_peer().

rust: configure MSRV in Clippy This setting makes Clippy never apply lints that are meant for later versions. In case the target precise toolchain is the one running, it does not make a difference, but this gives us a machine-parseable specification that is pretty standard. The README and `hg help rust` are updated to state that `clippy.toml` is the single source of truth about that, also lifting a minor ambiguity: it is fine if the MSRV is lagging behind the version in Debian testing.