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

@@ -182,3 +182,14 @@ def test_typify_integer_binops_in_floating_context():
 
     with pytest.raises(TypificationError):
         expr = typify(expr)
+
+
+def test_regression_typify_constants():
+    ctx = KernelCreationContext(default_dtype=Fp(32))
+    freeze = FreezeExpressions(ctx)
+    typify = Typifier(ctx)
+
+    x, y = sp.symbols("x, y")
+    expr = (-x - y) ** 2
+
+    typify(freeze(expr))  # just test that no error is raised