1.0.12.35: more safety -- less weakening of type checks

1.0.12.35: more safety -- less weakening of type checks

 * Weaken type check only if SAFETY < 2, and SAFETY < SPEED.  Thus
   SAFETY 2 becomes a "always full type checks".

 * Delete a stale comment above MAYBE-NEGATE-CHECK about weakening
   checks there -- this has not been the case since 0.7.9.41.

 * Fix type errors in target-sxhash.lisp that was masked by the
   weakening that used happen there.

 * Update commentary re. PROBABLE-TYPE-CHECK-P as per Alexey's email
   on sbcl-devel. Delete some other stale comments re. weakening.

 * Update the manual re. weakened type checks.