Use `for_each` in `Iterator::partition`
Use `for_each` in `Iterator::partition`

We already use this for `unzip`, but `partition` is not much different.

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