refactor: replace the memorizer from array to ES6 map in mobx.tojs()

[loatheb]

refactor: replace the memorizer from array to ES6 map in mobx.tojs() & extract the common cache method from recursion

Thanks for taking the effort to create a PR!

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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)

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[coveralls]

Coverage increased (+0.003%) to 92.972% when pulling 7d167ef on loatheb:refactor-tojs into dbe4c93 on mobxjs:master.

[mweststrate]

Thanks!