CSE does scalar replacement of aggregates

CSE does scalar replacement of aggregates

* module/language/cps/effects-analysis.scm (effects-clobber): New
  helper.
  (length): Only depend on &cdr.
  (synthesize-definition-effects!): New interface.

* module/language/cps/cse.scm (compute-available-expressions): Don't
  count out constructors here -- we'll do that below.
  (compute-defs): Add a comment.
  (compute-equivalent-subexpressions): Synthesize getter calls at
  constructor/setter sites, so that (set-car! x y) can cause a
  future (car x) to just reference y.  The equiv-labels set now stores
  the defined vars, so there is no need for the defs vector.
  (cse, apply-cse): Adapt to compute-equivalent-subexpressions change.