Add `"its"` modifier

[pokey]

• "take line air past end of its line"
• "move call air to its value" (to replace a return value with a nested function call in that return value)

Probably handle during inference phase?

Note that "past end of line" would want to change to "past end of its line", so this would be a breaking change. But it would make things more consistent, eg it would fix #571