Using named workers for blocks in a dataflow graph

[jclark]

This is to collect issues in using named workers to represent a dataflow graph.

Most important are

• Allow send actions that are conditionally executed #1273
• Support alternate receive action #1277

Also

• Inferred error type for named workers #45
• Ambiguity between single-wait-action and alternate-wait-action  #525
• Message send action should work with isolated object #784 (and then Blending readonly and isolated #1244)
• Named worker that is statically guaranteed not to outlive its function #1096
• Allow named worker to have on-fail clause #1274
• Interaction of wait-action and isolated #1275
• Interaction of receive-action and isolated #1276
• Support optional receive action #1284

[jclark]

We need to allow the error type of a named worker to be inferred #45. We originally had error<*> for this but there were some bad interactions with distinct, so we got rid of it for Swan Lake.

[jclark]

We need to way to say at compile time that a named worker does not outlive its function. This is useful when the function containing the workers is isolated: it would allow named workers to access structured parameters . This is #1096.