-O1 disables call precoloring

* module/language/cps/compile-bytecode.scm (compile-function)
  (emit-bytecode):
* module/language/cps/slot-allocation.scm (allocate-slots):
* module/language/cps/optimize.scm (cps-default-optimization-options):
  Allow the "lazy vars" optimization, a form of slot precoloring, to be
  disabled.  It will be disabled at -O0 or -O1, to speed compilation
  times.

module/language/cps/optimize.scm

@@ -130,4 +130,6 @@
    #:resolve-self-references? #t
    #:specialize-numbers? #t
    #:licm? #t
-   #:rotate-loops? #t))
+   #:rotate-loops? #t
+   ;; This one is used by the slot allocator.
+   #:precolor-calls? #t))