Add for each loops

[ecton]

The only hangup on implementing this is making it work with Map types. Ideally a for each loop over a map would yield a key and value, but we don't have a clean way for a function to return multiple values.

Early on in the design, the virtual machine allowed a function to return more than one value and for the returning function to receive more than one return value. It might be worth re-exploring that.