fhdl/decorators: make the transform logic more idiomatic

fhdl/decorators: make the transform logic more idiomatic

* the transformers work on classes and instances.
  you can now do just do:

    @ResetInserter()
    @ClockDomainRenamer({"sys": "new"})
    class Foo(Module):
        pass

  or:

    a = ResetInserter()(FooModule())

* the old usage semantics still work
* the old DecorateModule is deprecated,
  ModuleDecorator has been refactored into ModuleTransformer
  (because it not only decorates things)