rust: use native Meson support for clippy and rustdoc

rust: use native Meson support for clippy and rustdoc

Meson has support for invoking clippy and rustdoc on all crates (1.7.0 for
clippy, 1.8.0 for rustdoc).  Use it instead of the homegrown version; this
requires disabling the multiple_crate_versions lint (the only one that was
enabled from the "cargo" group)---which was not particularly useful anyway
because all dependencies are converted by hand into Meson subprojects.

rustfmt is still not supported.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>