Update geohash.h

The API has no reason to pass the range arguments by reference.  Doing so slows down the function.  
The arguments are not return values, and the first thing the code does is dereference them and copy them into stack variables.  Passing the input ranges by value skips this step and has fewer errors.  Furthermore, the arguments are small enough (2 primatives) that putting them onto the stack by value can make the function call inline even faster.