swift234

Add constexpr version of maximum to algorithm

Add the constexpr version of max which is only available in std since c++14.
Besides being a logical addition next to the already implemented constexpr version of min, there are actually other files, even in Swift/Basic itself, which re-implement this functionality, such as PrefixMap.h. Once this is implemented here, the functionality can be re-used in those other locations, instead of re-implemented each time.