lib.enum: allow using functional syntax for enum creation.

Fixes #910.

amaranth/lib/enum.py

@@ -118,7 +118,7 @@ def as_shape(cls):
             # replacement for `enum.Enum`.
             return Shape._cast_plain_enum(cls)
 
-    def __call__(cls, value):
+    def __call__(cls, value, *args, **kwargs):
         # :class:`py_enum.Enum` uses ``__call__()`` for type casting: ``E(x)`` returns
         # the enumeration member whose value equals ``x``. In this case, ``x`` must be a concrete
         # value.
@@ -130,7 +130,7 @@ def __call__(cls, value):
         # comparisons with enum members of the wrong type.
         if isinstance(value, Value):
             return value
-        return super().__call__(value)
+        return super().__call__(value, *args, **kwargs)
 
     def const(cls, init):
         # Same considerations apply as above.

tests/test_lib_enum.py

@@ -116,3 +116,6 @@ class EnumA(Enum):
                 r"shape used in bit vector context; define the enumeration by inheriting from "
                 r"the class in amaranth\.lib\.enum and specifying the 'shape=' keyword argument$"):
             Cat(EnumA.A)
+
+    def test_functional(self):
+        Enum("FOO", ["BAR", "BAZ"])