identify traits in libstd that should use associated constants

[pnkfelix]

as a subtask of #17307, once associated constants are implemented, we will want to update libstd to use them, rather than continuing to use methods to encode such constants.

A quick ack of the code base via ack --rust 'fn [^\(\)]*\(\) -> Self' shows the following candidate traits for being modified with associated constants:

• core::default::Default (but this is probably not a great idea; we probably want to continue allowing arbitrary code to be associated with constructing a default value)
• various numeric traits (but Gankro points out here that perhaps all of these numeric traits will be scrapped in our stdlib anyway)
  • core::num::Zero
  • core::num::One
  • core::num::Bounded
  • core::num::Float
• core::ptr::RawPtr<T>

[pnkfelix]

Blocked by #17841.

Nominating for P-backcompat-libs, not 1.0.

[pnkfelix]

Assigning P-high, not 1.0

[aturon]

Closing; this will fall out of the small amount of relevant remaining stabilization (mostly around the num traits).