An implementation of Group that allows that acts as a stack of Principals
with a single Principal Group member active at any time.
When one adds a Principal to a NestablePrincipal the Principal is pushed onto
the active Princpal stack and any of the Group methods operate as though the
Group contains only the Principal. When removing the Principal that corresponds
to the active Principal, the active Principal is popped from the stack and
the new active Principal is effectively set to the new top of the stack.
The typical usage of this class is when doing a JAAS LoginContext login
to runAs a new Principal with a new CallerPrincipal identity
without destroying the current CallerPrincipal identity and roles.