Auto merge of #5122 - acmcarther:acm-emit-features-from-cargo-metadata, r=alexcrichton

Auto merge of #5122 - acmcarther:acm-emit-features-from-cargo-metadata, r=alexcrichton

Emit Resolve.features_sorted in "cargo metadata"

This PR adds `features` to `metadata.resolve.nodes[*]` using the Resolve object's known features. This is different from the `features` fields that already exist elsewhere within metadata as this one is the actual features that need to be used in order to build the code.

Context: I'm currently using Cargo's internals to synthesize BUILD files for the Bazel build tool. `cargo metadata` currently provides almost everything I need in order to avoid using Cargo's internals -- *except* for this.

cc https://github.com/google/cargo-raze/issues/7