Consistent naming for invocation and execution

[andrewzhurov]

We define the following roles

Task adds two new roles to UCAN: invoker and executor. The existing UCAN delegator and delegate principals MUST persist to the invocation.

UCAN Field	Delegation	Task
iss	Delegator: transfer authority (active)	Invoker: request task (active)
aud	Delegate: gain authority (passive)	Executor: perform task (active)

Should it be
Invoker: requests invocation (active)
Executor: executes invocation (active)

Then, Invoker issues Invocation of a Task, as it is now, and executor issues Execution of an Invocation instead of Receipt.
I.e., renaming, in the upcoming release #21, Receipt -> Execution.

As a side-plus, "Execution Result", "Execution Effect" feels properly semantical.