Extend debugger to allow arbitrary evaluation in compiled code

Extend debugger to allow arbitrary evaluation in compiled code
environments.  This is done by the trick of rewriting SCode, replacing
references to compiled-code variables with combinations that do the
reference.

Fix bug in compiled-code frame parser.

Change `print-gc-statistics' to show info about constant and heap
space in use.

New operation `system-library-directory-pathname' provides standard
way to find run-time library files.