JS: jsSaturate should return a Saturated IR
In the JavaScript backend, all calls to jsSaturate (which replaces UnsatBlock constructors in Unsat.JStat but retains the unsaturated type) are soon followed by calls to satJStat (which performs a direct conversion of a saturated Unsat.JStat to a Sat.JStat). Since we never end up wanting a saturated Unsat.JStat, the two functions can be combined into one such that jsSaturate :: Unsat.JStat -> Sat.JStat.