hdl.ast: do not cast comparand to shape in Shape.__eq__.

This doesn't match how other Python comparison operators work.
E.g. `1 == int("1")` but `1 != "1"`.

amaranth/hdl/ast.py

@@ -133,13 +133,8 @@ class Shape:
             return "unsigned({})".format(self.width)
 
     def __eq__(self, other):
-        if not isinstance(other, Shape):
-            try:
-                other = self.__class__.cast(other)
-            except TypeError as e:
-                raise TypeError("Shapes may be compared with shape-castable objects, not {!r}"
-                                .format(other)) from e
-        return self.width == other.width and self.signed == other.signed
+        return (isinstance(other, Shape) and
+                self.width == other.width and self.signed == other.signed)
 
 
 def unsigned(width):

amaranth/lib/data.py

@@ -38,7 +38,8 @@ class Field:
 
     def __eq__(self, other):
         return (isinstance(other, Field) and
-                self._shape == other.shape and self._offset == other.offset)
+                Shape.cast(self._shape) == Shape.cast(other.shape) and
+                self._offset == other.offset)
 
     def __repr__(self):
         return f"Field({self._shape!r}, {self._offset})"