Allow implementing traits with statically uncallable methods

[Stebalien]

Basically, given:

trait Foo {
    fn foo(self) where Self: Sized;
    fn bar(&self);
}

The following is valid:

impl<T: ?Sized> Foo for T {
    fn foo(self) where Self: Sized {}
    fn bar(&self) {}
}

But the following isn't:

impl<T> Foo for [T] {
    fn foo(self) where Self: Sized { unreachable!() } // Should disappear at compile-time.
    fn bar(&self) {}
}

This means that, for example, Iterator can't be implemented types that are known to be DSTs, only types that may be DSTs. This is an inconsistent and an unnecessary limitation.

[nikomatsakis]

Dup of #20021

[nikomatsakis]

(I closed in favor of #20021 as it has more discussion)