iter: Implement .fold() for .cloned() and .map()
iter: Implement .fold() for .cloned() and .map()

Implement .fold() specifically for .map() and .cloned() so that any
inner fold improvements are available through map and cloned.

graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust