Build Mercurial with Rust
As this has become essentially mandatory in the final product [1], it is a potential source of trouble that HDK worskpaces are built without Rust.
On the other hand, this makes the initial build even more complicated than it is already, we should provide an option to bypass it, as it shouldn't make much difference for quite a lot of development tasks (e.g pure Ruby).
[1] technically no, it's the default, but falling back is complicated now that the presence of the nodemap file implies requirement of the Rust code)