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. 
Would it change something in the given example ? whether or not "c" is enabled, there would be a group boundary (either b-c or b-d).

It might be different if c is in group 1. There would be no group boundary when c gets enabled.

Thinking out loud here, I'll probably put this commit aside from the PR for now.