Feature/add back insert iterator (#603)
* Add back_inserter implementation
- Mainly a copy of the STL implementation found in LLVM.
- Add test_back_insert_iterator unit test.
* Add documentation and use ETL_OR_STD macro
* Add MIT License in header
* Move back_insert_iterator into iterator.h
* Remove unused code
* Strictly follow the C++ STL naming
https://en.cppreference.com/w/cpp/iterator/back_insert_iterator
Strictly following the C++ STL would have container_ be container.
* Make the check for C++11 clearer
* Run the unit test only for C++11
* Add front_insert_iterator
- Make back_insert_iterator available to C++03
- Add ETL_CONSTEXPR17,ETL_NODISCARD and ETL_USING_CPP11
- Replace std:move usage by etl::move
- Update doc
* Use explicit namespace for adressof() and move().
