Various fixes

#749 Code change causes warning with armcc compiler
#753 IAR's overload resolution struggles with etl::span::span(etl::array&)
#754 ambiguous etl::bit_stream_reader::bit_stream_reader() for some etl::span
#755 warning about constexpr in array.h
#756 Parameter pack compilation error
#758 Move assignment not working for non-empty containers
#761 Make "private" includes relative
#765 etl::optional cannot be constexpr
#766 etl::unique_ptr::reset() can call deleter with nullptr argument
Re-enabled bit_stream_writer tests