Remove `using` declarations from header files

[krismz]

Recommend removing all using declarations from header files and use the appropriate scope operator like std:: since this is a library that others will be including. It's best not to pollute the global namespace in this fashion as it could lead to unpredictable, hard-to-find errors in programs that include headers in this library. using declarations are fine in source files and other code that will not be #included by users.

Examples of lines to remove include:
using namespace std;
using std::min;