#657 Patch for removing `using namespace std;` from headers

[Alexander Riccio]

While investigating compilation-time-improvements, I noticed a bunch of using namespace std; in header files.

This is a big no-no, so I fixed it. This patch qualifies all uses of std:: functions/variables in headers, and most uses in .cpp files. In .cpp files where there are too many uses of std:: functions/variables, I've inserted a using std::vector; or using std::string; immediately after all the includes.

[Menno Vogels]

Could you elaborate on why it is a big no-no?

[Alexander Riccio]

A multitude of reasons. It's also almost always unnecessary, if just a few types/functions are needed, as for that, we can using std::vector.

[Menno Vogels]

Thanx

[Alexander Riccio]

Yeah, you can get away with it small projects/CS courses, but it's a ticking time bomb anywhere else.

[Don HO]

Alexander,

Could you make a souce zip for the merge?

Thank you,
Don