Auto merge of #33079 - bluss:split-iter, r=alexcrichton

Split core::iter module implementation into parts

Split core::iter module implementation into parts

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

+ mod (adaptor structs whose private fields need to be available both for them and Iterator
  + iterator (Iterator trait)
  + traits (FromIterator, etc; all traits but Iterator itself)
  + range (range related)
  + sources (Repeat, Once, Empty)