Support Authorization

[reid-spencer]

Security is a cross-cutting design element that should be factored into any design system.
It should be possible to specify RBAC style access constraints in the body of any Process
definition with syntax like:

role RWAccess is "A role that has no access to anything"
role ROAccess is "A role that allows read-only access"
context Foo is {
  accessible by ROAccess
  handler Writeables is {
    accessible by RWAccess
   ???
  }
}

So, Role is a new definition and definitions can specify who's allowed to access them. Applied at the context level,
it means every message in any handler in that context has those access privileges. In a handler, it means the
messages handled by that handler have those privileges. The principle of most specific specification wins is used.