* module/language/cps2/dce.scm: New file.
* module/language/cps2/optimize.scm: Enable CPS2 DCE pass.
* module/Makefile.am: Add language/cps2/dce.scm.

* module/Makefile.am (CPS2_LANG_SOURCES): Add effects-analysis.scm.
* module/language/cps2/effects-analysis.scm: New file, based on
  cps/effects-analysis.scm.
* module/language/cps2/utils.scm (intmap-map):
  (compute-defining-expressions, compute-constant-values): New helpers.

* module/language/cps2/simplify.scm (transform-conts): Return a persistent intmap.

* module/language/cps/intset.scm (intset-key-ranges, range-string):
  (print-helper, print-intset, print-transient-intset): New helpers.
  Install as intset printers.
* module/language/cps/intmap.scm (intmap-key-ranges, range-string):
  (print-helper): New helpers.
  (print-intmap, print-transient-intmap): Call the new helpers.

* module/language/cps2/utils.scm (worklist-fold): Add two-seeded arity.
  (worklist-fold2): Remove.

* module/language/cps2/renumber.scm (compute-tail-path-lengths): Adapt.

* module/language/cps/intmap.scm (intmap-fold): Add two-seeded arity.
* module/language/cps/intset.scm (intset-fold): Merge intset-fold2
  into this function, as a two-seeded arity.

* module/language/cps2/simplify.scm (compute-eta-reductions):
  (compute-singly-referenced-labels, compute-beta-reductions): Adapt
  intset-fold2 callers.

* module/language/cps/intmap.scm: Intmaps can now contain any value;
  #f does not indicate the absence of a value.  Instead we use a unique
  private sentinel to mark absent values or branches.
  (*absent*, absent?, present?): New helpers.
  (new-branch): Initialize empty elements to *absent*.
  (clone-branch-with-edit): New helper.
  (clone-branch-and-set): Use clone-branch-with-edit.
  (writable-branch): Use clone-branch-with-edit
  (empty-intmap): Initialize value to *absent*.
  (add-level): clone-branch-and-set doesn't take #f as a branch any
  more; use new-branch.
  (branch-empty?, make-intmap/prune, intmap-add!):
  (intmap-add, intmap-remove, intmap-next, intmap-prev):
  (intmap-fold, intmap-union, intmap-intersect): Use absent? to detect
  absent branches / values.
  (intmap-ref): Likewise.  Instead of returning #f if the value is not
  found, call the optional not-found procedure.  By default this will
  signal an error.

* module/language/cps/types.scm:
* module/language/cps2/renumber.scm:
* module/language/cps2/simplify.scm: Adapt to intmap-ref signalling an
  error by default if the value is not found.

* module/language/tree-il/compile-cps2.scm: Adapt to intmap-add
  signalling an error if #f was in the intmap as a value.

* module/language/cps2/utils.scm (fixpoint): Add two-argument arity.

* module/language/tree-il/peval.scm (<operand>): Rename "alias-value"
  field to "alias", which is now an operand and not an expression.
  This allows the operand to capture its environment; before, the
  alias was being visited in its use environment instead of its
  definition environment.
  (peval): Adapt to operand change.  Fix construction of rest bindings
  as well.
* test-suite/tests/peval.test ("partial evaluation"): New test.

* module/language/cps2/utils.scm (fixpoint): Fix embarrassing bug
  where it wouldn't actually fixpoint!  Didn't show up in practice
  because CPS2 hasn't run after contification yet.

* module/language/tree-il/compile-cps2.scm (convert): Fix bug
  in (apply - ...), because the instruction for "-" is "sub", and
  "sub" lookup was failing.  Caught by numbers.test.  Really I would
  like to get rid of $prim, somehow.

* module/language/cps2/optimize.scm: New file.
* module/language/cps2/simplify.scm: New file, factored out of
  simplify2.scm.

* module/language/cps/simplify2.scm: Remove, as it's obsolete.

* module/language/cps2/compile-cps.scm: Optimize the CPS.

* module/Makefile.am: Adapt for added and deleted files.