Fix the time consume problem in method instructionAtAddress

[wangnianwu]

If the address is not continuous, it will visit all integers from the current to target address(The length exceeds the map size), it causes taking too much time to call retrieveActiveBody in SootMethod.

[jpstotz]

As you have changed the Map type from HashMap to TreeMap wouldn't it be easier to use the TreeMap specific functions like floorKey or ceilingKey to get the nearest instruction to an address instead of iterating through the whole TreeMap to find the address?

[wangnianwu]

@jpstotz updated it by your advise