Experiment with skipping wrappers for native React components

This should make initial rendering a bit faster, as well as cases
where many components change simultaneously.

It should also lead to slightly less memory use.

The drawback is that native components no longer use
shouldComponentUpdate, and so requires more conversion from cljs
to js values – but this is now quite fast anyway.
Add todo
Remove obsolete code path
Tweak setup
Get rid of old wrapping code

Inputs are now backed by a proper Reagent component. The same class
is now shared by every input instance.
Merge branch 'master' into nowrapper
Fix mount ordering to be more robust
Some code cleanup
as-component -> as-element
Introduce create-element
Some refactoring
Remove helper for shouldComponentUpdate
Support meta keys in native elements
Add stuff to changelog