Fixed several issues in Wt::Dbo::collection<>

[skoehler47]

This pull request covers/fixes several issues with Wt::Dbo::collection<>:

• The collection::iterator operator == may cause an access violation due to a missing null-check in the third case; this can occur if impl_ == nullptr && other.impl_ != nullptr && other.impl_.ended_ == false, which is not covered by the first 2 cases.
• collection::iterator and collection::const_iterator are derived from std::iterator, which is deprecated in C++17. Since deriving from std::iterator does nothing but adding some type definitions, adding those typedefs directly to iterator/const_iterator solves the deprecation issue while being fully compatible with C++14 and before.
• const_iterator declares the assignment operator operator = (iterator), but the definition is missing. I added one which mimics the behavior of the const_iterator(iterator) constructor.
• There were some 32/64bit signed/unsigned type conversion issues in collection::size() and count() functions. I added some static_casts to resolve the numerous warnings caused by this, though i suggest a more thorough fix for the issue

[emweb]

Could you also reword your commits so that they have a short title and a longer description (with line breaks), instead of having the entire commit message on a single long line?

[skoehler47]

Done. I hope it is more to your liking now.

I usually prefer 'using' over 'typedef' too, but decided to go with typedef in this case to keep consistency with the rest of Wt::Dbo.

[emweb]

Yeah, I suppose the rest of Dbo is still in the old style, but I think for new code we can go ahead and move towards the new style.

I'll make some modifications on your branch. There are also some simple tests I want to add.

Regards,
Roel