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

[workspace]
members = ["hg-core", "hg-cpython", "rhg"]
exclude = ["chg", "hgcli"]
resolver = "2"