Default to suspending indefinitely

This enables typescript strict mode. This highlights two issues:

1) We have to use `Generator` instead of `Iterator`, since the `return` and `throw` functions are not mandatory for `Iterator`, and we're currently not doing anything sensible if they are absent.

2) The TReturn of a generator has to be `TOut | undefined`, because in the case of a halt, there is no sensible value to return from the generator.

Strict mode

Unfortunately using `Operation<unknown>` though more typesafe, rules out too many safe programs to be useful.

Very often we don't know and don't care about the return value of a task. Since we erase this type so often, let's just default it to `unknown` so we don't have to type it out all the time.