Documentation strings in src/{protocol,mixins,service,provider,package}.lisp

* src/protocol (find-service): added explanation of if-does-not-exist
  keyword parameter in documentation string
  (*services*): added documentation string
  (find-provider): likewise
  (setf find-provider): fixed missing word in documentation string
* src/mixins.lisp (header): updated copyright
  (name-mixin): added documentation string
  (documentation-mixin::documentation): likewise
  (documentation-mixin): likewise
  (provider-list-mixin): likewise
* src/service.lisp (header): updated copyright
  (standard-service): added documentation string
* src/provider.lisp (header): added one-line summary
  (class-provider::class): added documentation string
  (function-provider::function): likewise
* src/package.lisp (package service-provider): likewise
Renamed accessors of specialized providers in src/{provider,package}.lisp

* src/provider.lisp (class-provider::class): changed reader
  class-provider-class -> provider-class
  (make-provider t class-provider): adapted to renamed reader
  (make-provider-form t class-provider list t): likewise
  (print-object class-provider t): likewise
  (documentation class-provider eql t): likewise
  (function-provider::function): renamed reader
  function-provider-function -> provider-function
  (make-provider t function-provider): adapted to renamed reader
  (make-provider-form t function-provider): likewise
  (print-object function-provider t): likewise
  (documentation function-provider eql t): likewise
* src/package.lisp (package service-provider): added exported symbols
  provider-class and provider-function
Better make-provider-form in src/{protocol,provider}.lisp

* src/protocol.lisp (make-provider-form): added required parameters args
  and environment
  (make-provider-form symbol t t t): likewise
  (make-provider-form t symbol t t): likewise
* src/provider.lisp (make-provider-form t class-provider list t):
  likewise
  (make-provider-form t function-provider list t): likewise
Better print-object methods for providers in src/provider.lisp

* src/provider.lisp (print-object class-provider t): print the class
  name if it is different form the provider's name
  (print-object function-provider t): similar for function name
Exported class names in src/package.lisp

* src/package.lisp (package service-provider): added exported symbols
  standard-service, class-provider and function-provider
* test/protocol.lisp (test protocol.find-service.restarts): do not
  package-qualify `standard-service'
* test/macros.lisp (specialized-service): likewise
  (test macros.define-service.redefinition.change-class): likewise
  (specialized-class-provider): likewise
  (specialized-function-provider): likewise