Clean up atom constructor

[krawaller]

Here's a take on cleaning up the structure and return typings in atom.ts:

• extract inner basicAtom function
• extract inner derivedAtom function
• extract inner customAtom function
• remove atomConstructor
• collapse atom into simply returning basicAtom, derivedAtom or customAtom depending on input
• add file to test return types of atom (this "passes" both on master and on this branch)

No behaviour change, this is all about tidying up types and making the code easier to reason about.

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[krawaller]

@merisbahti I also see this as a preparatory step for the dev tools support; by having clearer atom construction code it will be easier to add eventual new parameters ("store" id, atom description, etc) needed for dev tools.