Yield statements aren't type checked

[od0x0]

I skipped over implementing type checking for yield statements. Functionally, these are almost like jumps from a value and spatial tag perspective, except the timeline may be different at either end. It might be a good idea to factor out the timeline manipulation as a separate splitpoint, and then it would almost exactly be a jump and closer to an inverted external cpu function invocation (more similar to a perform statement that appears with algebraic effects) except the continuation must be serialized.