1.0.20.6: smaller allocation regions & reduced pinning

 * Align objects at least one page in size on page boundaries.
   Previously only "large" objects (those at least 4 pages in size)
   were guaranteed page alignment.

 * Don't allow small objects to cross page boundaries.

 * The effect is to reduce the size of continuous allocation regions
   that start and stop on page boundaries. Since GENCGC conservativism
   operates on such regions, it's effects are reduced as well: for
   SBCL self build this reduces the number of pinned pages by ~45%.

 * Also report the amount of bytes found (in case of allocation
   failure) more accurately.