Feature/ranges part1

[whaeck]

This PR introduces a number of sfinae structures and aliases related to iterators and ranges.

This PR also introduces a ViewBase CRTP base class for views using these sfinae structures and aliases. The ViewBase assumes that the Derived class has a Derived::iterator type, a begin() and end() iterator. All methods with the exception of begin() and end() are made available depending on the type of the Derived::iterator type:

• if Derived::iterator satisfies concepts::IsInputIterator: operator== and operator!=
• if Derived::iterator satisfies concepts::IsForwardIterator: front(), size(), empty() and operator bool()
• if Derived::iterator satisfies concepts::IsBidirectionalIterator: back()
• if Derived::iterator satisfies concepts::IsRandomAccessIterator: operator[] and at()

The ViewBase interface should satisfy the C++20 view_interface, with the exception of data() which requires contiguous iterators (a concept introduced in C++20).

The ViewBase is now applied as the base class for the IteratorView which previously could only be used for random access iterators. It can now be used for all iterator types and the interface will be made available based on the underlying iterator type.

[timburculosis]

Looks good, just one code cleanup bit to consider.

[timburculosis]

I know we discussed this already, but after reading through this I have another idea. Consider combining the two SFINAE'd functions into one function with a static assert. Since there's only two functions, one of which that works and the other that fails to compile, you don't need the SFINAE in the template arguments anymore. The static_asserts should handle what you were originally using SFINAE for - the code won't compile if the static_assert isn't met.

[whaeck]

Done! No sfinae, I like it :-)