Lint usage of negative feature names in Cargo.toml

[kennytm]

(Only works with cargo clippy, may not be in scope with clippy. See #1614)

Features are supposed to be additive, and negatively-named features violate it. Lint when the crate declares a feature named no_std / no-std, and suggest using default-features = ["std"] instead.

(Main motivation in rust-lang/rfcs#1841.)

[clarfonthey]

Perhaps we could also suggest common conventions, like nightly => unstable.