Support multiple return types via auto and enhanced tuple expressions

[rsmckinney]

Support multiple return types via auto and enhanced tuple expressions.

For better readability omit the requirement for parenthesis for tuple expressions used as multiple return types.

auto result = aggregate();
int min = result.min;
int max = result.max;
int avg = result.avg;

auto aggregate() {
  return min(), max(), avg();
}

[rsmckinney]

See https://github.com/manifold-systems/manifold/tree/master/manifold-deps-parent/manifold-ext#multiple-return-values

Feature available with release 2022.1.16