mbed-ce@master + fixes + gcc 11 support (#135) ...

* Fix rtos::Mutex stub, add trylock() method

* LowPowerTicker - Add ::attach(...) method + call callback

* mbed_power_management - rename sleep --> mbed_sleep to avoid conflicts

Fixes conflicts w/ unistd.h and boost::ut

* GCC 11/C++ 20 - Remove deprecated type_traits

* GCC 11/C++ 20 - Remove redundant templating of methods

* Warning - pragma ignore invalid-noreturn for unit tests

* Fix astyle errors

* Fix missing mbed_sleep() calls

- Split mbed-core and mbed-rtos into -sources and -flags targets ...

- Fix some logic errors in top level cmakelists related to building greentea tests
- Fix mbed_create_distro() for OBJECT libraries

Deprecate instead of remove index_type

mstd::span rename index_type to size_type according to spec

Namespace scope mbed:: added for CriticalSectionLock used in Atomic templates

Fix make_span text in comment block

Fix last extra space issues

Backport dynamic extent when including <span> ...

Keep make_span functions even when <span> is available for compatibility

Fix missing defintion

Add to list of backports

Add some comment blocks

Add deduction guides

Remove nonstandard as_bytes

Fix minor style issues

Replace classes with structs where applicable

Use original <span> if present and the right version

Missed a pointer operator

Fix pointer and reference operator style

Fix newline at end of file

Update default initializers

Remove explicit constructors

Fix remaining namespaces and add include files

Remove c++17 inline of constexpr

Fix namespaces and span prototype

Prevent warning on subspan<0,N>

Add container constructor and make_span

Correct span constructors

Add iter_reference_t to iterator header

Fix endif comment

Adds basic span implementation