Lint against Sized constraints where appropriate

[UserAB1236872]

I've used Rust for a while now, and a long time ago struggled with needing to add : Sized to some types. Apparently quite a while ago, this changed to Sized being default and needing to request unsizedness in certain contexts. I had been struggling thinking

trait Foo {
    type Assoc;
    fn foo(&self, in: &Assoc);
}

struct Bar {}

impl Foo for Bar {
    type Assoc = [AType];
    fn foo(&self, in: &[AType]) { ... }
}

Had simply become impossible (I haven't needed it since pre-1.0 so it could've been true) until I went on the Rust Discord and had it revealed that, in fact, Sized is the default and ?Sized needs to be requested; an un-constraint rather than a constraint.

Unfortunately, I've been operating under this assumption for a while so I have some code with associated types that needs Sized behavior that explicitly calls for type Assoc: Sized or <T: Sized>. It might be worth linting against Sized constraints and noting it's effectively a non-constraint.

One consideration is that : Sized could potentially be seen as documentation (i.e. "hello future coder do not change this to ?Sized please or things will break"), so I don't know if it should be on by default or not.

Related to #1368.