As we start trying to use new facilities, we're likely to need some more
helpers.
In particular, ARM C 5 has no C++11 support in its library at all, so
to avoid totally breaking it we need some backup.
For the other toolchains, we can add a few C++17/C++20/TS extensions
into namespace mbed to make life a little easier.
* For ARM C 5: C++14 type_traits subset, std::move, std::forward,
std::array, std::initializer_list, std::begin, std::end,
std::align, std::maxalign_t, std::aligned_storage,
alignof + alignas macro replacements.
* For ARM C 5: MBED_CONSTEXPR_FN_14 and MBED_CONSTEXPR_OBJ_14 to
mark things that can only be constexpr in C++14 or later.
* For other compilers: mbed::void_t, mbed::type_identity,
mbed::conjunction, mbed::disjunction, mbed::negation,
mbed::experimental::nonesuch, mbed::experimental::is_detected family,
mbed::remove_cvref, mbed::as_const.
Kevin Bracey