Add new pass to optimize branch chains to table dispatch

* module/language/cps/switch.scm: New pass.

* module/Makefile.am (SOURCES):
* am/bootstrap.am (SOURCES): Add switch.scm.

* module/system/base/optimize.scm (available-optimizations):
* module/language/cps/optimize.scm (optimize-first-order-cps): Run
  switch optimization at level 2.

* libguile/hash.c (JENKINS_LOOKUP3_HASHWORD2): Add note regarding
  cross-compilation.

* module/language/cps/graphs.scm (intmap-select): New definition.
* module/language/cps/utils.scm (compute-singly-referenced-labels): Move
  here, from various places.  Doesn't take a body intset argument.

* module/language/cps/contification.scm:
* module/language/cps/closure-conversion.scm:
* module/language/cps/simplify.scm: Use compute-singly-referenced-labels
  from utils.

* module/language/cps/effects-analysis.scm (annotation->memory-kind*):
  (annotation->memory-kind): Add symbol annotation cases.

module/language/cps/graphs.scm

@@ -34,6 +34,7 @@
             intmap-keys
             invert-bijection invert-partition
             intset->intmap
+            intmap-select
             worklist-fold
             fixpoint
 
@@ -99,6 +100,12 @@ disjoint, an error will be signalled."
                   (intmap-add! preds label (f label)))
                 set empty-intmap)))
 
+(define (intmap-select map set)
+  (persistent-intmap
+   (intset-fold (lambda (label out)
+                  (intmap-add! out label (intmap-ref map label)))
+                set empty-intmap)))
+
 (define worklist-fold
   (case-lambda
     ((f in out)