New labels for issues and PRs

[ratijas]

I took the liberty to start triaging issues and PRs with labels, and since we are developing a Rust package, I naturally copied them from https://github.com/rust-lang/rust/labels, but not every other — only those essential ones which would be meaningful for our project's size, specificity and pace.

For example, most labels represent static info about an issue/PR which could be determined initially or right after it has been posted. The only real-time dynamic labels are from a subset of S- state labels:

• S-inactive
• S-waiting-on-author
• S-waiting-on-review
  And even for those I'm not entirely sure whether they would be useful without some sort of bot, since non-members probably won't be able to alter labels on their PRs — but hey, they can just re-request review with a single click, right?

So, currently there 35 labels, grouped into Areas, Categories, Type of bug, Operating Systems, Difficulty and Priority. Colors are consistent with rust-lang/rust repo.

"Call for participation" group of labels would allow to mark good first issues with E-easy, which addresses the request #131.

Any ideas and suggestions are warmly welcome!

[ratijas]

Also adding some Rust and C++ glue areas to the labels. They are not intended for wrappers in general, but rather for some language-specific features which make things work together.

For example, how about adding compile-time constexpr support? That would go into A-C++, I guess.