client.create_mutable_file(contents=) now accepts a callable, which is

client.create_mutable_file(contents=) now accepts a callable, which is
invoked with the new MutableFileNode and is supposed to return the initial
contents. This can be used by e.g. a new dirnode which needs the filenode's
writekey to encrypt its initial children.

create_mutable_file() still accepts a bytestring too, or None for an empty
file.