Fix typing of constants

This is an umbrella MR combining a few small changes:

• Fixes typing of constants. If a typed Constant inside of an untyped ConstantExpr was encountered, the typification used to fail (see new test).
• Strengthens test_typify_integer_binops now that arbitrary expressions can be deferred.
• Increases the information content of two error messages.
• Fixes a typo.

tests/nbackend/kernelcreation/test_typification.py

@@ -131,13 +131,11 @@ def test_typify_integer_binops():
 
     ctx.get_symbol("x", ctx.index_dtype)
     ctx.get_symbol("y", ctx.index_dtype)
-    ctx.get_symbol("z", ctx.index_dtype)
 
-    x, y, z = sp.symbols("x, y, z")
+    x, y = sp.symbols("x, y")
     expr = bit_shift_left(
-        bit_shift_right(bitwise_and(x, 2), bitwise_or(y, z)), bitwise_xor(2, 2)
-    )  #                            ^
-    # TODO: x can not be a constant here, because then the typifier can not check that the arguments are integer.
+        bit_shift_right(bitwise_and(2, 2), bitwise_or(x, y)), bitwise_xor(2, 2)
+    )
     expr = freeze(expr)
     expr = typify(expr)