feat(*): more performant lazy one-time binding support

BEAKING CHANGE:
Lazy-binding now happens on the scope watcher level.

What this means is that given `parseFn = $parse('::foo')`,
bind-once will only kick in when `parseFn` is being watched by a scope
(i.e. `scope.$watch(parseFn)`)

Bind-once will have no effect when directily invoking `parseFn` (i.e. `parseFn()`)