Change Detection: perf, fix, style

Edit: The 2 lasts commits are about improving perfs:
1. by using regular fns over getters/setters. The speed gain is impressive in FF, Chrome is a little bit slower but still much faster than FF,
2. by inlining `isPresent()` and `isBlank()` where there is no need to test for `undefined`

Note: benchpress results sometimes vary from a run to the next, especially with FF. 
Hard to say on Chrome, varying too much.

For Firefox, commit 4 gives a 1.4x improvement.