Add Iterator trait and for loop

[kodek16]

Add a trait named Iterator that enables iteration over arbitrary sequences. Also consider adding Iterable trait to be implemented by collections with a single iter method that returns a new Iterator.

As the main consumer of these traits, add a for loop syntax. for loops require an Iterable value. They create a new variable visible inside the loop body that takes on consequent values returned by the created Iterator.

The syntax for for loops could look like this:

var v = [1, 2, 3];
for (x in v) {
  writeln x;
}

v must implement Iterable, and the type of x is inferred from this implementation.