Make cloned values indistinguishable

Define "Returns a copy of the value" to mean a == b -> a == b.clone().

Original unworkable proposal (see discussion below):

Currently, clone explicitly doesn't guarantee that the following are always indistinguishable:

let a = f();
let b = g();
let c = b.clone();
h(a, b);

let a = f();
let b = g();
let c = b.clone();
h(a, c);

This is unfortunate. However, the trait does say that Clone::clone() "Returns a copy of the value." which could imply indistinguishably (i.e., a copy of a value is indistinguishable from the original). It would be nice to make this explicit (so that, e.g., a == b implies a == b.clone()).

/cc #1203

Start by reading the discussion on issue #1544 and the related issue #1203, focusing on the distinction between the original proposal and the current definition. Determine whether the Clone contract can be stated as shown and what RFC or Rust documentation location would record it; completion requires an agreed, unambiguous specification.