[FEATURE] Add __toString() and make Resource use more convenient
To use a Resource with file operations, one can use the
resource:// scheme and the __toString() method of any
To make this a little easier, this change adds two new
methods to the Resource class:
__toString() returns the SHA1 of the ResourcePointer the
Resource is holding (as is documented for ages).
getUri() returns a ready-to-use resource://<sha1> URI of
the ResourcePointer the Resource is holding.