* module/language/cps/dfg.scm: Refactor so that we can think about
building a dominator tree. Split continuations out of use maps and
put them in a separate table, which will have more flow information.
(visit-fun): Mark clauses as using their bodies.
(lookup-predecessors, lookup-successors): New exports.
(find-defining-expression): Add an exception for clauses, now that
clauses are in the flow graph.
(continuation-bound-in?): Rename from variable-bound-in?, as it can
currently only be used for continuations.
* module/language/cps/contification.scm (contify): Adapt to use
lookup-predecessors and continuation-bound-in?.
* module/language/cps/contification.scm (contify): Exhaustively replace
contified tail continuations, to fix a bug in nested tail-recursive
contifications. Likewise, call lookup-return-cont when searching for
common return continuations.
* module/language/cps/dfg.scm (variable-free-in?): Rename from
variable-used-in?, to match CWCC language.
(variable-bound-in?): New interface.
* module/language/cps/contification.scm (contify): Adapt caller. Add
more comments.
