...which was a legacy from the time when destructors were implemented using resource types.

Add `rc_buffer` lint for checking Rc<String> and friends

Fixes rust-lang#2623

This is a bit different from the original PR attempting to implement this type of lint.   Rather than linting against converting into the unwanted types, this PR lints against declaring the unwanted type in a struct or function definition.

I'm reasonably happy with what I have here, although I used the fully qualified type names for the Path and OsString suggestions, and I'm not sure if I should have just used the short versions instead, even if they might not have been declared via use.

Also, I don't know if "buffer type" is the best way to put it or not.  Alternatively I could call it a "growable type" or "growable buffer type", but I was thinking of PathBuf when I started making the lint.

changelog: Add `rc_buffer` lint

Downgrade rc_buffer to restriction

I think Arc\<Vec\<T\>\> and Arc\<String\> and similar are a totally reasonable data structure, as observed by others in the comments on [rust-lang#6044](rust-lang/rust-clippy#6044 (comment)) as well. Doing `Arc::make_mut(&mut self.vec).push(...)` or `Arc::make_mut(&mut self.string).push_str("...")` is a terrific and well performing copy-on-write pattern. Linting this with an enabled-by-default <kbd>performance</kbd> lint strikes me as an unacceptable false positive balance.

As of rust-lang#6090 the documentation of this lint now contains:

> **Known problems:** This pattern can be desirable ...

which should indicate that we shouldn't be linting against correct, reasonable, well-performing patterns with an enabled-by-default lint.

Mentioning rust-lang#6044, rust-lang#6090.
r? `@yaahc,` who reviewed the lint.

---

changelog: Remove rc_buffer from default set of enabled lints