Add associated items to impls and traits.

[otrho]

We should be able to add types and consts to trait and impl declarations, as per:

https://doc.rust-lang.org/reference/items/associated-items.html

Useful for:

impl u64 {
    pub const MIN: u64 = 0;
    pub const MAX: u64 = 18446744073709551615;
    pub const BITS: u32 = 64;
}

and obviously:

pub trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
}

• Associated consts in traits and abis: Support associated constants to traits and abis #3797
• Associated types in traits and abis: Support associated types for traits and abis #4487