Implement children keys and methods like react does

[dignifiedquire]

~~Still work in progress,~~ this comes from issues I was having around libraries assuming how the children methods behave.

I ported all tests from https://github.com/facebook/react/blob/master/src/isomorphic/children/tests/ReactChildren-test.js that were relevant.

Things left to do

• [x] Make all tests pass, currently 8 failing
• [x] Cleanup code and simplify as much as possible

Update tests are passing, skipping iterable support for now