SRFI-45: Support multiple values; add promise? predicate.

SRFI-45: Support multiple values; add promise? predicate.

* module/srfi/srfi-45.scm (eager): Accept any number of arguments.
  Store the list of arguments in the value record.  Previously, only one
  argument was accepted, and that value was stored in the value record.
  (delay): Support expressions that return any number of arguments.
  (force): Return the list of values stored in the value record.
  (promise?): Export.

* doc/ref/srfi-modules.texi (SRFI-45): Update docs.  Remove typing
  for simplicity in discussing multiple values.

* test-suite/tests/srfi-45.test: Add tests.  Add FSF copyright for 2010
  and 2013.  Add missing year to André van Tonder's copyright notice.