Optional support for zlib-based in-memory deflate/inflate for core files

 * As this is based on zlib, only add the dependency when
   :SB-CORE-COMPRESSION is enabled as a build-time feature.  On x86-64,
   compressed cores take about 1/4 the space, but start up in a few
   tenths of a second.

   Unlike gzexe'd executables, compressed images work without writing
   to /tmp.

   If :SB-CORE-COMPRESSION is enabled, trigger compression with the
   :COMPRESSION argument to SAVE-LISP-AND-DIE.

 * Also add a NEWS entry for the literal complex-single-float bugfix