Simplification to reactive stream subscription support.

Closes #492.
Allow promises to be used more than once.

Closes #480.
Peel back a layer of objects in the execution serialization machinery to reduce allocations.
Peel back more layers of objects in the execution serialisation.
Peel back even more layers of objects in the execution serialisation.
Peel back even even more layers of objects in the execution serialisation.