Make `numLocales` a `param` for `CHPL_COMM=none`?

[bradcray]

While talking with a user today, we were talking about coding idioms that would be more or less expensive when used in a single- vs. multi-locale setting. One idiom that was intriguing to me was:

var slots = someExpr % numLocales;

where he was wondering whether he would need to special case this part of the code to avoid the mod for a single-locale run. This made me wonder whether we make numLocales a param for CHPL_COMM=none compiles, where it seems that the answer is "no." But it seems like we could and that this could have some minor benefits, doesn't it?

[bradcray]

As a workaround in the meantime, we discussed creating routines:

proc numlocs() param where (CHPL_COMM=="none") {
  return 1;
}

proc numlocs() {
  return numLocales;
}

and using that in the modulus instead of numLocales above. We also discussed that if we had the param(bool) feature discussed in https://github.com/chapel-lang/chapel/issues/14639#issuecomment-565529472, that could be used as a workaround more succinctly than creating the two overloads above.

[bradcray]

A challenge to doing this is that we currently implement, and indicate to the user, that numLocales is a config const. Of course, it's not a very configurable one in the CHPL_COMM=none case, so there would be some effort in the documentation as well as the coding to explain this clearly.