(InputPort) Update immediately when assigned a constant value

There are times when we want an input port's value to be assigned immediately (usually for some sort of constant value) rather than as part of a graph update. However, the syntax for this is currently weird:

writer->setFlag = true; // currently evaluated with graph.update()
writer->setFlag.post(true, true) // immediately assigned to the port's target

This proposes to always call immediate=true when assigning a constant-value to a port.