Removing "using namespace std"

[Rhialto]

Would you be open to a merge request that removes using namespace std; from ./src/libpar2internal.h? It will also mean to add std:: to all occurrences of string, set, map etc etc, so it is somewhat of a churn. On the other hand, using namespace std; is frowned upon for a long time. The work is not hard but a bit annoying, so I'm asking before I'm actually doing it.

The reason I'm asking is that there is another project (http://gitlab.com:Rhialto/nget) that has incorporated an old copy of par2, and I had to clean its use of std:. Unfortunately, via an include file, the using still leaked into nget code. So I cleaned up the copy, but I might as well offer the same upstream. And I want to look into updating the copied version.

The change would be quite similar to this commit, I'd imagine: https://gitlab.com/Rhialto/nget/-/commit/e305f2d44c6536f72b95084259cac04b50043857

There are a few more cleanup commits I did in the copy of par2; after this I would like to propose them here as well.