Implementation and test cases for the R6RS (rnrs records inspection)

Implementation and test cases for the R6RS (rnrs records inspection)
library.

* module/Makefile.am: Add module/rnrs/records/6/inspection.scm to RNRS_SOURCES.
* module/rnrs/records/6/inspection.scm: New file.
* module/rnrs/records/6/procedural.scm: Assorted refactoring:
    Create index constants for record, rtd, and rcd field indexes;
    record-type-vtable, record-constructor-vtable: More informative display
    names;
    (make-record-type-descriptor): fold left, not right when creating vtable;
      store field names as vector, not list;
      detect opaque parents
* test-suite/Makefile.am: Add test-suite/tests/r6rs-records-inspection.test to
  SCM_TESTS.
* test-suite/tests/r6rs-records-inspection.test: New file.