NewEnvelope creates a new envelope, starting a weavelet subprocess (via child.Start) and establishing a bidirectional connection with it.

Envelope starts and manages a weavelet in a subprocess.

InProcessChild is a fake envelope.Child that represents the in-process weavelet.

Options contains optional arguments for the envelope.

ProcessChild is a Child implemented as a process.

Child manages the child of an envelope.

EnvelopeHandler handles messages from the weavelet.