Added RPN version of 24 game

An alternate implementation of the 24 game problem which uses RPN (reverse polish notation expressions). 

FYI [RosettaCode](http://rosettacode.org/wiki/Reports:Tasks_not_implemented_in_Rust) still shows that 24 game and 24 game solve are not yet completed.
Yup! This makes it concise enough to not require a separate function call. Made it inline! Thanks! :+1: