[suggestion] use ES6 map instead of array in mob.toJS() memorizer?

[loatheb]

I have a Idea:

• What problem would it solve for you?
• Do you think others will benefit from this change as well and it should in core package (see also mobx-utils)?
• Are you willing to (attempt) a PR yourself?

This is a suggestion of refactoring tojs method.

I looked at the code and found that in tojs comment stated that it hope to improve the performance of the cache through the ES6 Map. Also, I extract a common function in recursion.

PR is here 👉 1517

Can anyone do a review or give me some suggestion of it?

[mweststrate]

Reviewed, will be part of next release, thanks!