compiler letrec_check pass: recognize effect-free primitives

As in "Fixing Letrec". This improvement corrects a performance
regression with the revised expansion of R5RS `letrec`, which
wraps right-hand sides with `values`.

Besides detecting effect-free primitives, we have to fix the
treatment of the right-hand side for a multi-binding `letrec-values`
clause. For now, we conflate all of the bindings in a single
clause.
clarify comment in JIT implementation

Refines commmit c0ec9702e8.