registerDelay not implemented on the non-threaded runtime

Even if the non-threaded runtime doesn't provide tools to implement it as efficiently, it's still possible to implement registerDelay with correct semantics. That would be a lot more pleasant to work with than an exception at run time.

On a side note, that current behavior isn't even mentioned in the haddocks, which makes it even more surprising.

Locate registerDelay and the non-threaded runtime entry point, then read how the threaded runtime defines its semantics. Implement equivalent behavior without the current runtime exception, and update the Haddocks to describe the non-threaded behavior; done means the semantics are correct and documented.

Written by the indexing model from the issue text.