This involved adding the Fs() method to DirEntry as it is needed in
the metadata mapper.
Unspecialised fs.Dir objects will return a new fs.Unknown from their
Fs() methods as they are not specific to any given Fs.
- Change rclone/fs interfaces to accept context.Context
- Update interface implementations to use context.Context
- Change top level usage to propagate context to lover level functions
Context propagation is needed for stopping transfers and passing other
request-scoped values.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
