further compilation fixes -- all files compile fine now

further compilation fixes -- all files compile fine now

* ice-9/runq.scm (strip-sequence): Remove use of obtuse guile `define'
  extension.

* ice-9/boot-9.scm (while): Redefine so as not to unquote in a procedure.
  Less hygienic. Perhaps we should switch to syncase at some point.

* ice-9/session.scm (help): Redefine as a normal macro, so that it can be
  compiled. Not very useful though -- further effort should go into
  (system repl ...).
  (system-module): Removed, it didn't work, and is not useful as far as I
  can tell.

* ice-9/string-fun.scm (string-prefix-predicate): Remove guile define
  extension usage. Compilation also fixed by `while' compilation fix.

* ice-9/threads.scm (par-mapper): Remove guile define extension usage.