Auto merge of #4382 - Xanewok:expose-filter-rule, r=alexcrichton

Expose ops::cargo_compile::FilterRule

I've been mindlessly copying and consuming the Cargo API to work on the build plan prototype for RLS and I noticed that, it seems, pub `ops::cargo_compile::CompileFilter` is exported, but not the `ops::cargo_compile::FilterRule`, which `CompileFilter` uses, as a part of a public API.

If I'm wrong and missing something, feel free to ignore/close it. It was possible to match/destructure `CompileFilter` before, but with this change it's possible to use the underlying type.