Consuming lens implementation from @microstates/lens

[taras]

In the process of building Microstates, @cowboyd wrote our own implementation of lenses. It turned out to be super tiny. I published it as a separate package available as @microstates/lens. This PR removes the local implementation in favour of the external package.

[cowboyd]

I'd like to make some changes to the lens library first that I've been meaning to make for a really long time. For example, the childAt function isn't really related to the defineChildren function, and so while the former is critical to lenses, the later isn't. Been meaning to break them up for a while.

[coveralls]

Coverage decreased (-0.09%) to 98.752% when pulling 8f70c54 on tm/external-lenses into cd4114b on master.