style: code cleanup

@mhevery this is the last "cosmetic" PR before the serious work can start tomorrow. I hope I can do the updates I now have in mind w/o altering performances, we'll see.

I've added some todos in `PrototypeMap` that I'll fix tomorrow.

And sorry, this PR re-layouts `if/else` on multiple lines because I started some of it before your previous feedback and I've picked the multi-line version during conflict resolution _- believe it or not, I find them more readable :)_. But I can revert tomorrow if requested, no problem.