Fix disassembler glitch on x86-64.

If a disassembly terminated at a #x48 byte not as part of a complete
instruction, the labelling pass left a random REX prefix in the state
which the display pass erroneously picked up.
Remove obsolete comment
Export unquoting COMMA readers from SB-INT, and trivial cleanup.
Preserve quasiquotation in SB-CLTL2:MACROEXPAND-ALL
Remove inapplicable comments re formerly flaky test, since fixed.
fix find-defstruct-description's errorp flag
Fix first half of lp#1354606 for splicing in quasiquoted vectors.
Fix undefined behavior case of lp#1354606
Fix lp#1354623 - macroexpansion of QUASIQUOTE broken by rev.88411384
Transform (STRING sym) to (SYMBOL-NAME sym)
Fix WITH-PACKAGE-ITERATOR for empty list of packages
":IMPORTANT NIL" in DEFTRANSFORM now means really unimportant.

Users aren't likely to be able to make code more efficient when
shown "The first argument is a T, not a (NOT NULL)" so don't.
(It only worries them)
Document some bugs in package manipulation.
Improve DOCUMENTATION tests

* Use new helper function ASSERT-DOCUMENTATION

* Consistently use WITH-TEST

* Group all DOCUMENTATION tests in one section of the file
DOCUMENTATION works on instances of FUNCALLABLE-STANDARD-CLASS

Previously, methods on DOCUMENTATION and (SETF DOCUMENTATION) were
specialized on STANDARD-CLASS, excluding FUNCALLABLE-STANDARD-CLASS.

While this was in line with the method signatures listed in the CLHS
entry for DOCUMENTATION, that entry also states (for DOC-TYPE equal to
TYPE):

  If x is a symbol, returns the documentation string of the class whose
  name is the symbol x [...]

indicating that (documentation CLASS-NAME 'type) should work for all
kinds of classes.

Specializing the four methods on CLASS instead STANDARD-CLASS is
1) sufficient to enable operation on FUNCALLABLE-STANDARD-CLASS
2) safe since the accessed %DOCUMENTATION slot is located in CLASS
Extract the state machine of WITH-PACKAGE-ITERATOR into a function.

Also, by avoiding POSITION-IF and FIND-SYMBOL, not to mention the
useless (THE HASH-VECTOR) everywhere, it's significantly faster.
Add WITH-TEST, improve test for DOCUMENTATION on (SETF ...)
Test DOCUMENTATION for DOC-TYPEs PACKAGE, VARIABLE
Apply constraint-propagate-if info even if the then branch has none

This wasn't an issue when the else branch always received the negation
of the then branch, but constraint-propagate-if can adjoin arbitrary
constraints.
Fix instance-length discrepancy in genesis again.

MAKE-COLD-LAYOUT lacked the math to ensure that an instance header
specifies an odd number of words following. As such it differed
from what the target runtime does.