rust-overlay: make it work as a drop-in replacement for nixpkgs' rustPlatform

[steveej]

The easiest way to use the overlay would be if one could just override the whole rustPlatform derivation, so that rust programs within nixpkgs will automatically make use the overlay.

I stumbled upon this when debugging why racer doesn't use a given RUST_SRC_PATH, finding that the wrapper overrides it from the rustPlatform's rust sources: https://github.com/NixOS/nixpkgs/blob/eb9f76911f1fee6ebd519220bc9843d3adac5ee7/pkgs/development/tools/rust/racer/default.nix#L27

[Ericson2314]

I think some parts of this / rust-nightly-nix can be upstreamed to nixpkgs, which is a) good in general, and b) make this easier.

[P-E-Meunier]

As an extension of this issue, it would be very cool indeed if targets used the same syntax, for instance.