Lint to warn against unnecessary extra hashes in raw string literals

[SabrinaJewson]

What it does

Checks for the use of more hashes that are necessary in r#…#-style literals, and emits a warning about it telling you to use fewer hashes. Suggested name: needless_raw_string_hashes; suggested categories: style or pedantic.

Advantage

• Code is kept more consistent; there are fewer ways to express one piece of logic.
• Code automatically self-documents the level of “rawness” needed in the string literal.

Drawbacks

• Sometimes for alignment purposes one might want to add more hashes than necessary.
• Maybe this should be in rustfmt instead?

Example

let s = r##"Hello "world"!"##;

Could be written as:

let s = r#"Hello "world"!"#;

[Centri3]

Yeah this seems pretty reasonable, @rustbot claim