Some things don't implement IntoIterator

[Veedrac]

There are some missing IntoIterators:

• By-reference and mut-reference for Option
• By-reference and mut-reference for Result
• By-reference for Path
• Several misc. internal things (TraitObligationStackList, SearchPaths, ArchiveRO, RawTable, Book)

Given the by-value IntoIterator impls for Option and Result, I don't see why by-reference iteration would be special. Path should probably be IntoIterator. I'm indifferent with regards to internal stuff.

[apasel422]

This was partially addressed by #28039.

[arielb1]

internal things will only get impls when they are needed for internal purposes. e.g. TraitObligationStackList is only ever accessed by immutable reference.

[apasel422]

CC @rust-lang/libs. The only outstanding part of this is &Path (and presumably &PathBuf).

[alexcrichton]

Seems like an easy bug to knock out!