Improve the function/constant/property interface pipeline

[katsaii]

What is your feature request?

Something like

Catspeak.interface.__tryGetConstant__ = function (data) {
  if (data.name == "thing1") {
    data.value = 1;
  } else if (data.name == "thing2") {
    data.value = 2;
  } else {
    return false;
  }
  return true;
};

which the compiler will call to search for a constant value.

Please describe in detail how you expect this new feature to behave.

Right now using all the weird and wacky expose... methods causes slowdown if a user wants to expose everything. Having greater control over exactly how the Catspeak compiler interprets free variables will be nice for users who don't care about safety.