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.
Set upper memory limits for the perf tests.

Hopefully this stabilises CI.
Remove the half done execution snapshot stuff.
Create a renderer for promises.

Closes #486.
Create a renderer for publishers.

Closes #509.