Remove the zero checking of the heap.

It's been several months without any issue after adding the lazy
zeroing of heap pages, so it's probably safe to remove the zero
checking of the heap pages.  It can still be enabled by setting the
appropriate variables.
Allow stack-tn's to be accessed in the float arith vops.

The vops can directly access stack tn's now instead of having to load
it into a register first.
Simplify the macros that generate the basic float operations.
Update from commit logs.
Import lisp-unit.  This is the version from quicklisp which marked it
as lisp-unit-20130615-git.  No changes.
Add lisp-unit as a module.  Use (require :lisp-unit) (or "lisp-unit")
to load lisp-unit.  This also allows the regression tests to be
self-contained so we don't have to have lisp-unit installed via
quicklisp or some other means.

Unlike asdf and defsystem, we do not provide a precompiled fasl.

code/module.lisp:
* Add defmodule forms for lisp-unit

contrib/load-lisp-unit.lisp:
* Module file to compile and load lisp-unit.
Add news about lisp-unit.