sim: use Format.* for VCD output, remove hdl._repr.

This also changes `decoder` a bit: when an enum is used as a decoder,
it is converted to a `Format.Enum` instead. The original enum is still
stored on the `decoder` attribute, so that it can be propagated
on `Signal.like`.

amaranth/back/rtlil.py

@@ -5,7 +5,7 @@ import io
 from ..utils import bits_for
 from .._utils import to_binary
 from ..lib import wiring
-from ..hdl import _repr, _ast, _ir, _nir
+from ..hdl import _ast, _ir, _nir
 
 
 __all__ = ["convert", "convert_fragment"]