Moved contents of {src,test}/*-mixin.lisp -> {src,test}/location-mixins.lisp

* src/append-nodes-mixin.lisp: removed; contents has moved to
  src/location-mixins.lisp
* src/create-missing-nodes-mixin.lisp: likewise
* src/ignore-empty-results-mixin.lisp: likewise
* src/location-mixins.lisp: new file; contains mixins classes for
  location classes
* test/append-nodes-mixin.lisp: removed; contents has moved to
  test/location-mixins.lisp
* test/create-missing-nodes-mixin.lisp: likewise
* test/ignore-empty-results-mixin.lisp: likewise
* test/location-mixins.lisp: new file; contains unit tests for
  location mixin classes; use `ensure-cases' for multiple cases
* xml.location.asd (system xml.location): removed files
  src/append-nodes-mixin.lisp, src/create-missing-nodes-mixin.lisp and
  src/ignore-empty-results-mixin.lisp; added file
  src/location-mixins.lisp
  (system xml.location-test): removed files
  test/append-nodes-mixin.lisp, test/create-missing-nodes-mixin.lisp
  and test/ignore-empty-results-mixin.lisp; added file
  test/location-mixins.lisp
Improved documentation in src/protocol.lisp

* src/protocol.lisp (header): added one-line summary
  (loc): improved documentation string
Use uninterned symbol in in-package form in src/singleton-location.lisp

* src/singleton-location.lisp (in-package xml.location): use
  uninterned symbol to designate package