Add "transient" intmap interface
* module/language/cps/intmap.scm (make-atomic-reference):
(get-atomic-reference, set-atomic-reference!): New helpers.
(*branch-size-with-edit*, *edit-index*): Branches now have a trailing
field, an atomic reference to their owner.
(<transient-intmap>): New record type. A mutable intmap.
(new-branch): Set the "edit" field on the branch.
(clone-branch-and-set): No editor for this field.
(assert-readable!, writable-branch): New helpers.
(transient-intmap, persistent-intmap): New exported functions.
(intmap-add!): New function.
(intmap-next, intmap-prev, intmap-ref): Work on transient intmaps.
(intmap-fold): Persist the intmap before folding over it.