Introduce "_p.asEngine()" method to return an accessor with preset "__engine" accoount and role

[shimono]

Overview

Introduce a new accessor creation method "asEngine()" in the engine library and make
the following code return an accessor with pseudo role "../__/__engine" and pseudo account "__engine"

  _p.asEngine().cell().

Token spec

Engine should then return an access token with the following spec:

Role	Subject
box/_engine	{CellURL}#__engine
../__/_engine	{CellURL}#__engine

With these modifications, box install alone can enable engine to access any box-local resource by distributing ACL to grant "__engine" to access that resource.

Related Core modification

personium/personium-core#431

considerations.

• Is it necessary to accept pseudo-account "__engine" as a "serviceSubject" attribute in the ESC (Engine Service Collection) setting?
• Do we need another pseudo-role, pseudo-account "__rule" for event-rule-triggered execution of engine script?