feat: add equals option to observable.box (resolve #1580)

[fi3ework]

Thanks for taking the effort to create a PR!

If you are creating an extensive PR, you might want to open an issue with your idea first, so that you don't put a lot of effort in an PR that wouldn't be accepted. Please prepend pull requests with WIP: if they are not yet finished
PR checklist:

• Added unit tests
• Updated changelog
• Updated docs (either in the description of this PR as markdown, or as separate PR on the gh-pages branch. Please refer to this PR). For new functionality, at least API.md should be updated
• Added typescript typings
• Verified that there is no significant performance drop (npm run perf)

Try to add equals to the options of observable.box (#1580).
Changelog & docs update will be updated later 😀

[coveralls]

Coverage increased (+0.006%) to 93.678% when pulling 728a829 on fi3ework:equals-for-box into c74c9f8 on mobxjs:master.

[mweststrate]

Thanks for the PR! Looking good, will make some minor performance improvements and then release

[mweststrate]

Merging as part of #1874. Thanks!

[mweststrate]

Fixed and released in 4.9.0 / 5.9.0