Clang-tidy errors

[PeterZhizhin]

Hello! There seems to be a lot of clang-tidy errors when I copy-paste tqdm into my project.
There is a (not complete) list of them:

1. error: 'tqdm::Tqdm<tqdm::RangeIterator<unsigned int> >' has virtual functions but non-virtual destructor.
2. error: 'tqdm::MyIteratorWrapper<tqdm::RangeIterator<unsigned int> >' has virtual functions but non-virtual destructor
3. A bunch of Declaration uses identifier '_Iterator' which is a reserved identifier.

Consider running clang-tidy on the project and fix all the errors. There are some significant issues like non-virtual destructors and returning copies from operators instead of returning references.