Renamed system service-provider -> architecture.service-provider

* src/protocol.lisp (header): changed one-line summary
* src/conditions.lisp (header): likewise
* src/mixins.lisp (header): likewise
* src/macros.lisp (header): likewise
* src/package.lisp (header): likewise
* test/protocol.lisp (header): likewise
* test/macros.lisp (header): likewise
* test/package.lisp (header): likewise
* architecture.service-provider.asd: renamed service-provider.asd ->
  architecture.service-provider.asd
  (package architecture.service-provider-system): renamed
  service-provider-system -> architecture.service-provider-system
  (system architecture.service-provider): renamed service-provider ->
  architecture.service-provider
  (system architecture.service-provider-test): renamed
  service-provider-test -> architecture.service-provider-test
Added designator methods, Improved designator types in src/{types,protocol}.lisp

* src/types.lisp (service-designator): disallow nil; added
  documentation string
  (provider-designator): likewise
* src/protocol.lisp (service-name symbol): new method; lookup
  designator and delegate to service object
  (service-providers symbol): likewise
* test/protocol.lisp (test protocol.service-accessors.designators):
  new test; check passing designators to accessors