compilation passes return third value: the continuation environment

* module/system/base/compile.scm: Expect compile passes to produce three
  values, not two. The third is the "continuation environment", the
  environment that can be used to compile a subsequent expression from
  the same source language. For example, expansion-time side effects can
  set the current module, which would be reflected appropriately in the
  continuation environment.

* module/language/assembly/compile-bytecode.scm:
* module/language/bytecode/spec.scm:
* module/language/ecmascript/compile-ghil.scm:
* module/language/ghil/compile-glil.scm:
* module/language/glil/spec.scm:
* module/language/objcode/spec.scm:
* module/language/scheme/compile-ghil.scm:
* module/system/base/compile.scm: Update compile passes to return a
  continuation environment.

module/language/assembly/compile-bytecode.scm

@@ -40,7 +40,7 @@
               (get-addr (lambda () i)))
        (write-bytecode assembly write-byte get-addr '())
        (if (= i (u8vector-length v))
-           (values v env)
+           (values v env env)
            (error "incorrect length in assembly" i (u8vector-length v)))))
     (else (error "bad assembly" assembly))))