Add IsUpToDate function for SharedStrings

[lacrosse91]

Hi. Addresses #1842 and add IsUpToDate function for SharedStrings. This PR is for a performance change.

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[lacrosse91]

@googlebot I signed it!

[tst-lsavoie]

Thanks for the PR @lacrosse91. The code looks good, but we'll need to do some testing before we merge it.

[lacrosse91]

Thanks for review, @tst-lsavoie. I would appreciate if you could tell me what I have to do.

[tst-lsavoie]

@lacrosse91 you don't need to do anything right now. I want to do some testing on my end before giving a final approval.

[lacrosse91]

Thanks for reply. I'm very happy to contribute to this great project.