bzrlib should provide context managers for locking

In a recent Launchpad branch I implemented this:

class ReadLockTree(object):
    """Context manager to claim a read lock on a bzr tree."""

    def __init__(self, tree):
        self.tree = tree

    def __enter__(self):
        self.tree.lock_read()

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.tree.unlock()
        return False

And used it like this:

  with ReadLockTree(tree):
      tree.do_something_even_trow_exception_or_return

It would be great if it was generally integrated into bzrlib so that it can be used like this:

  with lockable_object.read_lock():
      lockable_object.do_something_even_trow_exception_or_return

Same for write locks.