globally unique marks and labels using syntax-session-id

globally unique marks and labels using syntax-session-id

* libguile/macros.c (scm_init_macros): Add definition of new
  syntax-session-id helper.

* module/ice-9/psyntax.scm: Capture a reference to syntax-session-id at
  boot time.  Uniquify marks and labels using the session id.

* module/ice-9/boot-9.scm: Shunt syntax-session-id off to (system
  syntax) once we finished booting.

* module/ice-9/compile-psyntax.scm: Override syntax-session-id when
  remaking psyntax to avoid spurious diffs.

* module/ice-9/psyntax-pp.scm: Regenerate.