FusedIterator is a marker trait that promises that the implementing
iterator continues to return `None` from `.next()` once it has returned
`None` once (and/or `.next_back()`, if implemented).

The effects of FusedIterator are already widely available through
`.fuse()`, but with stable `FusedIterator`, stable Rust users can
implement this trait for their iterators when appropriate.

This was never established as a convention we should follow in the 'More
API Documentation Conventions' RFC:

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md

This trait can be used to avoid the overhead of a fuse wrapper when an iterator
is already well-behaved.

Conforming to: RFC 1581
Closes: #35602

split iter.rs into a directory of (implementation private) modules.

+ mod Adaptor structs
  - Private fields need to be available both for them and Iterator
+ iterator (Iterator trait)
+ traits (FromIterator, traits but Iterator itself)
+ range (range related)
+ sources (Repeat, Once, Empty)