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.
The API is as per posix standard, but does not provide stats for file/directory.
Stats buffer (block size, total block count, free block count) is filled for
entire mounted filesystem.
Required for other representations of FileSystems, ie LocalFileSystem
Introduces FileSystemHandle for the same behaviour as FileHandle and
DirHandle.
Requires the following to hook into file/dir lookup:
```
int open(FileHandle **file, const char *filename, int flags)
int open(DirHandle **dir, const char *path)
```
This hook is provided by the FileSystem class, so requires no changes
from implementations.
Martin Kojtal
deepikabhavnani
Christopher Haster