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.

rust-readme: mentioned that format check is enforced by CI

rust-readme: mentioning clippy especially since there is a CI check for it.

rust-readme: rst fixes

rustdoc: nodemap doc refreshing Not pretending to be comprehensive. - correcting some inconsistencies - adding a few missing doc-comments - adding more cross references (in some cases it's right beside the current documentation item, but it will nevertheless also be useful, because `rustdoc` will warn us if inconsistencies arise).

rustdoc: fixed or introduced crossrefs in nodemap.rs

rustdoc: summary line for hg_path_to_os_string The main motivation of this change is to avoid the TODO being the summary line, even though this leads to a pretty obvious summary. Then doc-comments for the other functions are introduced for consistency.