refactor($interpolate): optimize watched $interpolate functions performance

This PR improves the performance of watched $interpolate functions: http://jsperf.com/ng-interpolation-optimization/8

This is the related Google groups thread, I explain my PR there: https://groups.google.com/forum/#!msg/angular-dev/QmcwYVkfsbo/8Y_pMsal0kYJ

If any changes are unclear, point them out and I'd be happy to elaborate on them :)

Signed the CLA as Rodric Haddad