Add add-standard-as-possible version of C++11 <mutex>.
A lot of the stuff in there is generic, but the actual mutex classes and
call_once need to interface with the OS.
For those, they're not available in ARMC5 or IAR; retargetting would be
necessary for ARMC6 and GCC, and I've yet to investigate how whether
that's possible. So for now I'm using local implementations.
Although `Mutex` in principle could support `timed_mutex` and
`recursive_timed_mutex`, we don't have `chrono` for the time parameters,
so hold off for now.
For the generic stuff like mstd::unique_lock, they are aliased to
std::unique_lock where possible.
Regularise the C++ support glue, adding `<mstd_type_traits>` etc.
These include the base toolchain file, backfill `namespace std` as much
as possible for ARM C 5, and then arrange to create unified
`namespace mstd`, which can incorporate toolchain bypasses
(eg `mstd::swap` for ARM C 5, or `mstd::atomic`), and include local
implementations of post-ARM C++14 stuff.
All APIs in `namespace mstd` are intended to function as their
`namespace std` equivalent, and their presence there indicates they
are functional on all toolchains, and should be safe to use in
an Mbed OS build (including not unreasonable memory footprint).
See README.md for more info.
Kevin Bracey