1.0.17.4: support for dynamic-extent structures

1.0.17.4: support for dynamic-extent structures

 * Replace %MAKE-INSTANCE-WITH-LAYOUT with %MAKE-STRUCTURE-INSTANCE,
   which has an IR2 transform that can handle both initialization and
   allocation of the structure. On x86 and x86-64 it can initialize
   all slots, whereas on other platforms it only does the layout and
   non-raw slots. (See RAW-INSTANCE-INIT/* below.)

 * EMIT-INITS needs two new kinds of inits to handle: :SLOT for
   instance slots, and :DD for the defstruct-description/layout.

 * DEF-ALLOC doesn't anymore use a simple boolean for denoting
   variable length allocation, but instead a keyword: either
   :VAR-ALLOC, :FIXED-ALLOC, or :STRUCTURE-ALLOC.

 * New VOPs: RAW-INSTANCE-INIT/* for all raw slot types, which are
   almost identical to RAW-INSTANCE-SET[-C]/* VOPs, except that they
   always have a constant index and do not return a result. Structures
   with raw slots can be stack allocated only on platforms that
   implement these VOPs, denoted in make-config.sh by the
   :RAW-INSTANCE-INIT-VOPS feature. ...we really could use a
   *VM-FEATURES* or something.