rust: add explicit resolver field to top-level cargo manifest file
Since Rust 1.51.0, Cargo has support for a better dependency resolver.
Workspace packages (like ours) need to explicitely set the field, so let's.
https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html
= Mercurial XXX =
== New Features ==
== Default Format Change ==
These changes affect newly created repositories (or new clones) done with
Mercurial XXX.
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
* remove the experimental infinite push extension
== Internal API Changes ==
== Miscellaneous ==