Fix path names / namespace names

[richard-vock]

I just wrote a conan package [1] for your library and noticed that the namespaces and include folders are prefixed by your github username. This is highly uncommon [2][3] and results in the necessity to fix the paths per file using search and replace tools.

Suggested fix:

Use the project name ("rangesnext") as the main namespace name and have include files in either ./include or ./include/projectname like all the other libraries.

[1] https://github.com/richard-vock/conan-rangesnext

[2] https://google.github.io/styleguide/cppguide.html#Namespace_Names

[3] https://codingart.readthedocs.io/en/latest/cpp/Naming.html#namespace