Allow Symbol keys in observable maps (fixes #1925)

[pimterry]

• Added unit tests
• Updated changelog
  • Not clear what the 'right' way to do this is, seems lots of PRs ignore it? I've added an 'Unreleased' section ready to rename for the next release for now.
• 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
  • No changes necessary imo
• Added typescript typings
  • No types have changed
• Verified that there is no significant performance drop (npm run perf)

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@mweststrate you mentioned safeStringify in #1925, I couldn't find that - looks like it's a function in the mobx-state-tree codebase?

I've written a separate key stringification function instead. Here it actually doesn't use JSON.stringify, because JSON can't represent symbols, and they're normally omitted from JSON stringified objects. For the same reason, I've also preserved symbol-ness in toPOJO here, instead of stringifying them, which feels correct, and helpfully means we get the correct behaviour (symbol keys omitted) in toJSON here too.

[coveralls]

Coverage increased (+0.006%) to 93.877% when pulling 9f12565 on pimterry:symbol-map-keys into 4451340 on mobxjs:master.

[pimterry]

@mweststrate any thoughts on this one? I'd love to get this fix in.

[mweststrate]

@pimterry looking great! Merging and releasing. I think this should be pack-ported to mobx4 too?

[mweststrate]

Backported, released as 4.9.4 / 5.9.4

[pimterry]

Amazing, thank you!