Lints about new methods

[mcarton]

Fix #618.

Edit: hum, should we allow:

impl Foo {
    fn new(..) -> Result<Foo, Error> { .. }
}

too?

[llogiq]

Yeah, we probably should allow Result<Self, _>.

[mcarton]

Yup, there are some examples in rustc (eg. term::terminfo::TerminfoTerminal::new(out: T) -> Option<TerminfoTerminal<T>>). I’ll do that tomorrow.

[llogiq]

Nice.

[llogiq]

Good stuff. 👍