fix(OsValues): Add ExactSizeIterator implementation

[tbu-]

It seems like an oversight that this was omitted.

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This change is 

[mention-bot]

@tbu-, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kbknapp to be a potential reviewer.

[kbknapp]

Awesome, thanks!

It looks like the Pinned nightly failed due to not being at least a 1.27 based nightly? Would you mind pinning a newer nightly in the .travis.yml?

Once the newer nightly is pinned, it should pass all lints from running cargo build --features lints (which just runs Clippy) to ensure there aren't new clippy lints which fail on that nightly (hence the reason for pinning a specific Nightly compiler version).

[tbu-]

The nightlies of 2018-06-22 don't work (rust-lang/rust#51699).

[tbu-]

OK, don't know how to find a working nightly.

[kbknapp]

No worries! Usually it just involves fixing the things clippy is warning about. I can take a look and push some commits this weekend ;)

[kbknapp]

Here is a relevant tweet that may be applicable here...looks like it wasn't just us

https://twitter.com/RustStatus/status/1010240719527739392?s=19

[tbu-]

Build successful! \o/

[kbknapp]

Thanks for this!