Bug 7003: jam: Improve API and test specifications

* sources/lib/jam/jam-ir.dylan
  (<jam-arg>): Define as including <string> rather than <byte-string>.

* sources/lib/jam/jam-state.dylan
  (jam-variable, jam-variable-setter): Allow variable names to be general
   instances of <string>.
  (jam-rule, jam-rule-setter): Allow rule names to be general
   instances of <string>.

* sources/lib/jam/jam-evaluator.dylan
  (jam-invoke-rule): Allow rule names to be general instances of <string>.

* sources/lib/jam/jam-reader.dylan
  (jam-read): Allow the input to be a general instance of <string>.

* sources/lib/jam/tests/jam-test.dylan
  (module-spec jam): Do not use unexported types in the defined
   signature of jam-target-bind.