Move component children into its attrs

The decision to make a :children field on components the values passed to
component's constructor has caused non-trivial incidental complexity.

I propose that we move the children given to component's constructor into the
component's attributes.

Pros:
*   Simplicity and consistency in getting/setting built children
*   Ability to get/assoc children with normal Clojure operations
*   Removes need for IBuiltComponent protocol and its implementations
*   The "children" passed to a component constructor are just as much attributes
    as any ordinary attribute; they are not guaranteed to be the children or even
    exist in the component's render output.
Cons:
*   Slightly more tedious to obtain children

I think the benefits greatly outweigh the (only) reasoning for not having
children within attrs -- especially given the incidental complexity that we've
solved around (IBuiltComponent) and future implications

/cc @scottrabin