Add support for coroutines

[ecton]

This is going to require the ability to temporarily swap the executing stack.

We may want to move "step" and all of its related functionality to a new type that receives a mut slice of the current stack. The Function type can have a stack and a current instruction that the interpreter uses when present and jumping into a function.

We can use the fairly-standard yield keyword: yield <Expression> ('to' <Expression>)?.

I'm sure it will end up being a little more complicated than this, for example, how to support the more verbose coroutine functionality alongside the simplified generator interface.