Modularize <iterator>

[miscco]

This modularizes the <iterator> header as done by libc++.

This is a precursor to updating to a more recent state of the code and bringing in C++20 ranges machinery

[wmaxey]

#0 0.498 In file included from /libcudacxx/libcxx/include/__iterator/advance.h:16,
#0 0.498                  from /libcudacxx/libcxx/include/iterator:681,
#0 0.498                  from /libcudacxx/libcxx/include/memory:661,
#0 0.498                  from /libcudacxx/libcxx/include/any:84,
#0 0.498                  from /libcudacxx/libcxx/src/any.cpp:9:
#0 0.498 /libcudacxx/libcxx/include/__iterator/iterator_traits.h:16:10: fatal error: concepts: No such file or directory
#0 0.498    16 | #include <concepts>
#0 0.498       |          ^~~~~~~~~~
#0 0.498 compilation terminated.
#0 0.500 [ 34%] Building CXX object libcxx/src/CMakeFiles/cxx_shared.dir/iostream.cpp.o
#0 0.507 In file included from /libcudacxx/libcxx/include/__iterator/advance.h:16,
#0 0.507                  from /libcudacxx/libcxx/include/iterator:681,
#0 0.507                  from /libcudacxx/libcxx/include/memory:661,
#0 0.507                  from /libcudacxx/libcxx/include/any:84,
#0 0.507                  from /libcudacxx/libcxx/src/any.cpp:9:
#0 0.507 /libcudacxx/libcxx/include/__iterator/iterator_traits.h:16:10: fatal error: concepts: No such file or directory
#0 0.507    16 | #include <concepts>
#0 0.507       |          ^~~~~~~~~~
#0 0.507 compilation terminated.

There's more errors in the log, but if you use the GCC-12 docker profile you can see that building the libcxx project fails.

[griwes]

Minor comments only, ship it.