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.