Shrink Atom and Reaction using a bitfield

[peterm-canva]

Following on from #3880, use a bitfield to reduce the size of Atom and Reaction. Large apps can have 10s or 100s of thousands of these. Also further shrink ComputedValue by storing diffValue in the bitfield, because it's only ever 0 or 1.

Memory Savings

Class	Bytes before	Bytes after
ComputedValue	100	96
Atom	48	40
ObservableValue (inherits from Atom)	76	68
Reaction	72	56

Name Changes

Note this PR changes the names of some members of the above classes to get around the problem mentioned here: #3880 (comment). If these are part of a documented, stable public API then we will need to find another solution.

Soaking Time

We discussed here #3880 (comment) that we should let the previous changes to ComputedValue soak for a bit to flush out any issues. We have been shipping those changes for a few weeks in production now and haven't seen any problems yet.

Code change checklist

• N/A Added/updated unit tests
• N/A Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

[changeset-bot]

🦋 Changeset detected

Latest commit: 281179f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
mobx	Patch

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[mweststrate]

Looking great again, thanks a lot!

[wh1sp3r]

My project stopped working after this update.
No error in output.

Will check exactly, what's happening

[urugator]

@wh1sp3r could be related to #3918

[spion]

I'll try to make a repro case this weekend and if it succeeds send a PR over to bump the version.