Format self types to a canonical form when possible

[nagisa]

struct Peach;

impl Peach {
    fn apple(self: Self) {}
    fn banana(self: &Self) {}
    fn chaenomeles(self: &mut Self) {}
}

all of these receivers have a more canonical form which ~everybody uses:

struct Peach;

impl Peach {
    fn apple(self) {}
    fn banana(&self) {}
    fn chaenomeles(&mut self) {}
}

This kind of thing can come up if refactoring the receiver to no longer use one of the smart pointers, or perhaps somebody considers this syntax to be more consistent (which it is, but in a typical codebase which typically utilizes the conventional form it'll stick out as a sore thumb)