Add rust_plugin rule to support Rust compiler plugins

[davidzchen]

Add support for using Rust compiler plugins.

Because plugins are an unstable feature, #14 would also make this easier to use for developers.

[acmcarther]

Does this issue also track proc-macro dependencies? Those have been stabilized (as of 1.15 IIRC).

Based off of rustc --version and https://github.com/rust-lang/rfcs/blob/master/text/1566-proc-macros.md, I think they're basically just another kind of rust_library, and can be specified with --crate-type. We'll need special handling if one of these is received as a dependency though.

[acmcarther]

Update on this:

This is actually supported trivially, and rustc links these identically to any other kind of dependency. Support for general crate types (#38 extended with "proc-macro" mapping to rlib, blocked on me), is all that is necessary.

[acmcarther]

This issue is obsolete:

As detailed in the prior comment, no special rule variant is required to support compiler plugins -- just a crate type. Likewise, compiler plugins can be included directly as normal dependencies and used (e.g. for custom derive).