SB-SEQUENCE:DOSEQUENCE variable capture fixes
New function SB-PCL:ENSURE-CLASS-FINALIZED

Simplifies lots of cases in which a particular aspect of a class can
only be accessed if the class has been finalized.
Improvements in methods on CHANGE-CLASS

* Use new function ENSURE-CLASS-FINALIZED

* New functions CHANGE-CLASS-TO-METAOBJECT-VIOLATION for error signaling
  and CHECK-NEW-CLASS-NOT-METAOBJECT for precedence list walking reduce
  code duplication
Improvements in CHANGE-CLASS tests

* (CHANGE-CLASS :SMOKE): check that objects cannot be change into
  metaobjects.

* (CHANGE-CLASS FORWARD-REFERENCED-CLASS): check that instances of
  FORWARD-REFERENCED-CLASS cannot be changed into other metaobjects.
MAP calls SB-SEQUENCE:MAP for extended sequences
CONCATENATE calls SB-SEQUENCE:CONCATENATE for extended sequences
MERGE calls SB-SEQUENCE:MERGE for extended sequences
Documentation and news entry for SB-SEQUENCE:{MAP,CONCATENATE,MERGE}