constexpr support for etl::unaligned_type + etl::traits namespace

Added constexpr support for etl::unaligned_type.
Added etl::traits namespace containing traits mirroring many ETL macros.
Traits are const for C++03, constexpr for C++11 and above.
Removed some uses of GCC builtins due to compatibilty with constexpr.
etl::swap is now ETL_CONSTEXPR14.
Changed ETL_ENDIANNESS_IS_CONSTEXPR to ETL_HAS_CONSTEXPR_ENDIANNESS.
Changed ETL_CONSTEXPR17 to ETL_CONSTEXPR14 for reverse iterators
Added template wrappers around memcpy, memmove, memcmp, memset & memchr.
The counter in etl::debug_count is atomic, if available.
Embedded essential UnitTest++ source into the project, as it is no longer maintained on GitHub.