* module/system/base/target.scm (target-runtime): New export.
* module/language/cps/optimize.scm (make-cps-lowerer): Load a
backend-specific lowering module dynamically.
* module/language/cps/guile-vm.scm: New module for lowering to Guile's
VM.
* module/language/cps/guile-vm/loop-instrumentation.scm:
* module/language/cps/guile-vm/lower-primcalls.scm:
* module/language/cps/guile-vm/reify-primitives.scm: Move here, from
parent dir.
* am/bootstrap.am: Update for new file list.
