Dyno: Iterate over param enum ranges

[DanilaFe]

Closes https://github.com/Cray/chapel-private/issues/7404.

This PR enables param iteration over ranges of enums. For instance, the following program produces one warning for every element in letters:

enum letters {a, b, c, d, e, f}

for param c in letters.a..letters.f {
  compilerWarning(c:string);
}

Curiously the original reproducer in https://github.com/Cray/chapel-private/issues/7404 does not work as expected, because warnings are not emitted from inside main. Not sure why that's the case, but it seems to be independent of param iteration.

While there, this PR resolves signedness concerns (preventing overflow etc.).

The high-level outline of the change is to think of iteration over enums as iteration over the indices of their elements. This is similar to, but not the same as, the enum's int values (not the same because users can change the values of enums by assigning them different integers, like red = 42).

Testing

• dyno tests
• motivator passes
• paratest
• paratest --dyno-resolve-only

Reviewed by @benharsh -- thanks!

[benharsh]

Could/should these two "populate" functions be part of a factory method on one of these structs?

[benharsh]

Would the logic in this branch be better placed in a factory method?