"Jar" for Rust: single file crate support for `rustc`

[Nemo157]

@mathstuf

neither do proc macros (e.g., graphql_client_codegen reading a query/schema file).

This could be supported with RFC 3200.

@epage

One possible problem is IDEs doing "go to definition".

Even vim is capable of showing files within archives, I would certainly hope that more modern tooling is too.