perf(scope): 10x. Share the child scope class.

This change causes Scope.$destory to run 10x faster. I suspect
Scope.$new is significantly faster as well, but I didn't measure it.
perf(compile): 1.8x speed up. Remove debugging information.

This is not complete, I am sure there are other places where we
attach debugging information to nodes.  These are the places that
effect ngRepeat benchmarks the worst.