Minor wire_struct improvements:

* The concatenated value can be set with the special `kwarg` "_value". This is
  useful when the `wire_struct`'s class name is not known.

* Changes to make `wire_structs` documentable with Sphinx. The generated class
  copies documentation from the decorated class (`__doc__`), and the generated
  class's name now matches the decorated class's name. Previously all
  `wire_structs` were named `_WireStruct`.

* Add a new `class_name` argument to `wire_matrix`. This lets users optionally
  set the name of the generated class. Previously all `wire_matrix` were named
  `_WireMatrix`.

Author: fdxmw