refactor($interpolate): remove some unecessary logic
refactor($interpolate): move standalone functions to the service scope
refactor($compile): check nodomevents at compile time instead of link
perf($compile): only re-$interpolate attributes values if the value has changed
perf($compile): caching $interpolate operations on isolated scope @ properties
perf($compile,ngModel,form): avoid invoking $interpolate for empty/falsy values