1.0.28.30: DEFGLOBAL, ALWAYS-BOUND, GLOBAL, SYMBOL-GLOBAL-VALUE

1.0.28.30: DEFGLOBAL, ALWAYS-BOUND, GLOBAL, SYMBOL-GLOBAL-VALUE

 * ALWAYS-BOUND allows the compiler to elide boundness checks
   for symbol value access, and prohibits MAKUNBOUND. This is
   handled via a new globaldb entry.

   GLOBAL makes the compiler elide TLS checking for symbol
   values access, and prohibits rebinding. This is handled via
   new globaldb :variable :type, namely :global.

   DEFGLOBAL is build on top of these. Global variables are mainly an
   efficiency measure on threaded builds, but can also express
   intention as they prohibit rebinding.

 * Add %SET-SYMBOL-GLOBAL-VALUE, FAST-SYMBOL-GLOBAL-VALUE, and
   SYMBOL-GLOBAL-VALUE VOPs to all backends. On unithreaded
   builds these are trivial copies of the non-global versions.

 * Tell SB-CLTL2 about both GLOBAL and ALWAYS-BOUND declarations too.

 * Document in the Efficiency chapter of the manual.

 * Prohibit declaring constants special.

 * Tests.

  Later: use these new features inside SBCL itself. For now there is
  only a single DEFGLOBAL used, but more could well be.