auto merge of #10622 : Kimundi/rust/str_de_iter, r=alexcrichton
This PR removes almost all `_iter` suffixes in various APIs of the codebase that return Iterators, as discussed in #9440.
As a summarize for the intend behind this PR:
- Iterators are the recommended way to provide a potentially lazy list of values, no need to name them painfully verbose. If anything, functions that return a specific container type should have more verbose names.
- We have a static type system, so no need to encode the return value of a constructor function into its name.
Following is a possibly incomplete list of all renamings I performed in the codebase. For a few of them I'm a bit unsure whether the new name still properly expresses their functionality, so feedback would be welcome:
... not showing all identical renamings for reverse versions
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I'm also planning a few more changes, like removing all unnecessary `_rev` constructors (#9391), or reducing the `split` variants on `&str` to a more versatile and concise system.