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.