Improved compilation scheme for enums.

Now that we allow choosing values for enum constants, there are two
compilation schemes for enums (i.e., data types where no constructor
takes arguments).
- C11 enums, i.e., `typedef enum { A = 0xff, ... } t;`. These are
  mandated to have type `int` in C11; C23 allows choosing the width, but
  we don't yet assume C23. We assume `sizeof int == 4` and error out if
  any of the user-chosen constants exceed the range of `int`.
- Macros. We cannot represent arithmetic on enum constants, so there are
  no issues about integer promotions, or a constant being sign-extended
  for the wrong reasons. We just print out integer literals, and try the
  first type that fits all possible values for the enum type, with a
  preference for unsigned types.

This fixes AeneasVerif/eurydice#123

Author: msprotz