Support using enum values in requires

[BenjaminLawson]

It would be nice to be able to use enum values in requires expressions:

enum PageScan:
   MIN = 1
   MAX = 100

struct Example:
  0 [+2] PageScan page_scan
    [requires: PageScan.MIN < this < PageScan.MAX] 

[reventlov]

After thinking about this a bit, I think there are two real options:

1. Support casts from enum types to integer.
2. Support the full set of comparison operators on enums.

I'm leaning towards option 2, which means moving the type checks for the <, >, <=, >= operators over to the same function that type checks == and !=. I think that's the only change needed: the way the various comparison operators are implemented in C++ and for inline evaluation should already work with enums.