More platforms, iterator and copy-constructor support for map, queue and array

0.6.0 (2021-02-09) New platforms, iterator and copy-constructor support.

• Support for iterators and copy-constructors in ustd::array, ustd::queue, and ustd::map.
  (Thanks @proddy for iterator sample implementation, fixes #4 .)
• Platforms: Feather M4 added: __FEATHER_M4__
• Blackpill STM32F411 added.
• Arduino Nano 33 BLE added. Note: requires platformio's lib_ldf_mode = chain+ definition
  in platform.ini, otherwise platformio will get confused about including WiFi.h, because
  of faulty #ifdef parsing of ustd_platformio.h.
• ASSERT() macros removed due to clashes.