Array Wapper, new XOR checksum, constexpr support.

Added etl::array_wrapper to add a zero cost STL like interface to pre-declared C arrays.
https://www.etlcpp.com/array_wrapper.html

Added etl::xor_rotate checksum algorithm.
https://www.etlcpp.com/hash.html

Added definition of ETL_CONSTEXPR to support constexpr when available.
https://www.etlcpp.com/macros.html