Overhaul features-status-dump

[Kat-zorn]

features-status-dump was made to dump the state of all features in the compiler and standard library to a JSON file. This allows out-of-tree programs to inspect the data, without needing to have access to the in-tree crates used to obtain the data.

After a certain RustNL talk inspired me to look through the list of features in search of interesting ones to work on, I decided to make a tool to make looking through them easier. Since this tool requires access to the Rust source to work, and will likely only be used by rust-lang devs, I decided to develop it in-tree. This crate seems to be the perfect basis to turn into this tool, as the following changes to it allow it to work for feature-finding, while retaining exact backwards compatibility (if I did it right):

• Only force at least one of --library-path and --compiler-path to be specified, so users can look through just the project they're interested in. Notably library lookup is much slower than listing language features. Being able to only look at language features is a massive performance advantage to users who only need to know those.
• Make --output-path optional and default to stdout, so users can get quick feedback.
• Add --before and --since flags for specifying a version range for the feature's since attribute.
• Add --unstable, --accepted, and --rejected flags for filtering by feature level.
• Add a --tracking-issue flag for filtering on presence of a tracking issue.
• Add a --format flag to choose whether to print human-readable plaintext or JSON.
• Add a --sort-by flag to see relevant features first. (Sorts by newest/oldest version, with issue number as tie breaker.)

I hope that the tool is welcome in this project, and will turn out to be useful. If there's any feedback, I'd be happy to hear it, as I'm quite new here.

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