Provide a minimal version of rust-overlay without manifests

[9cxndy]

... and un-deprecate fromManifestFile.

When using fenix, I developed a pattern where I add Rust manifest as a flake input, then create Rust toolchain with fenix's fromManifestFile. This enables me to use change the version of the Rust used while having a fixed version of fenix.

Now I am trying to migrate to rust-overlay (mainly because fenix is slow at addressing the latest nixpkgs breakage), I found two problems when I tried to do the same thing:

• fromManifestFile is deprecated here.
• rust-overlay repo includes Rust manifest files for the past two years, so the download size is unnecessarily large esp. considering I don't even use the manifests from rust-overlay.

I think it would be nice if there is a minimal version of rust-overlay that doesn't include the manifests.

Thanks!