POLA array.length Atom

[grithin]

Adding an array.length Atom to make observation of array.length as expected.

1751937
This is the change I was recommending in #3988

Performance tests pass. The only tests that don't pass, apart from the tests that fail prior to my change, are those related to Spy changes.

If this is an acceptable change to mobx, I will update the Spy unit tests and send a pull request.