Include guards around os-specific string elements.

[stevepentland]

This seems to be something missed some time ago as the original annotation for this 'use' was only feature debug. However this broke builds using that feature on Windows for a while.

This change uses guards both for the feature and also wasm/windows specific targets/environments.
These guards are the same as are currently in use for Windows/wasm in parser.rs

closes #1270 and is done in response to Drops-of-Diamond/diamond_drops#59

fyi: @jamesray1

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This change is 

[mention-bot]

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

[stevepentland]

I cannot locally reproduce the 1.20.0 failure, but it seems to be coming out of the unicode-normalization crate that is a dependency of our dev depencency version-sync

Edit: After looking at the travis build history, it appears this failure started happening about 3 days ago

[jamesray1]

LGTM! @ChosunOne

[ChosunOne]

Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[jamesray1]

Obviously the build needs to pass for 1.20.0 before merging.

[stevepentland]

Obviously the build needs to pass for 1.20.0 before merging.

Yeah, but as I mentioned in my previous comment, the failure seems to be unrelated to this particular change and looks to have started about 3 days prior to this PR. The first time it failed was during this run.

[jamesray1]

OK yeah fair enough, all the errors are coming from unicode-normalization-0.1.6/src/tables.rs.

[stevepentland]

It seems @kbknapp has been pretty quiet for a bit so we'll just have to wait for him to get back and offer some guidance on fixing the issue. The last change that went into master was merging #1244 which was passing when it came in, so it seems like something external may be responsible.

[kbknapp]

Sorry all, I was out for a while with my day job. I'm taking a look at this and will merge shortly 😉

[jamesray1]

Thanks! Let me know when this is in a release so that I can update the version in cargo.toml.