Add $branch expression type

Add $branch expression type

* module/language/cps.scm ($branch): New expression type; will replace
  $kif.

* module/language/cps/arities.scm:
* module/language/cps/closure-conversion.scm:
* module/language/cps/compile-bytecode.scm:
* module/language/cps/cse.scm:
* module/language/cps/dce.scm:
* module/language/cps/dfg.scm:
* module/language/cps/effects-analysis.scm:
* module/language/cps/primitives.scm:
* module/language/cps/renumber.scm:
* module/language/cps/self-references.scm:
* module/language/cps/simplify.scm:
* module/language/cps/slot-allocation.scm:
* module/language/cps/type-fold.scm:
* module/language/cps/types.scm:
* module/language/cps/verify.scm: Adapt to $branch expression type.