Add `.iter` method on `Range`

- [ ] `for i in a_range::from(1).up_to(5).iter() { let _i: &i32 = i; }` compiles
- [ ] Tests added as doc tests on `iter()` method

Start by locating the Range implementation and the existing test setup, then read how its current range methods expose values. Add the iter entry point described in the issue and verify the two checklist conditions, including the doc test on iter().

Written by the indexing model from the issue text.