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

rust/Cargo.toml

@@ -1,3 +1,4 b''
 [workspace]
 members = ["hg-core", "hg-cpython", "rhg"]
 exclude = ["chg", "hgcli"]
+resolver = "2"