Improves core method performances

Theses are small changes but I find out that when I load my app and display an heavy custom collection `SC.objectForPropertyPath` is call about 174757 times.

From my benchmark, with : 
`SC.typeOf`: 174757 => 653ms
`typeof`: 175382 => 453ms

Which makes the method 1.4 times faster and make me win 200ms.

With `tupleForPropertyPath` there is a gain of 25ms when executed about 173121 times.