IAR compilation fails at `_fs = { 0 };` due to
```
[Error] FATFileSystem.cpp@285,0: [Pe029]: expected an expression
```
Changing that object initializer list instead, as that seems
to pass IAR compilation, too.
Changed original memset to object initialiser list based on review
feedback from Kevin Bracey.
Macro which restricted compilation to GCC_ARM is removed.
Existing read_write() test is amended to call stat() and check that correct size is returned.
Adding new modules inside the namespace could be breaking change for existing code base
hence add `using namespace::class` for classes newly added to mbed namespace to maintian
backwards compatibility.
MBED_NO_GLOBAL_USING_DIRECTIVE is added to remove auto-addition of namespace
Macro guard `MBED_NO_GLOBAL_USING_DIRECTIVE` is added around namespace, to avoid
polluting users namespace.