Store the unique number of the class in the instance at allocation.

In preparation for using the instance class unique number for generic
function dispatch we store the unique number of the class in the
instance as soon as possible, i.e., when the instance is allocated,
either by ALLOCATE-INSTANCE or ALLOCATE-BUILT-IN-INSTANCE.

This is an absolute must, because the next thing after allocation is a
call to INITIALIZE-INSTANCE which is a generic function, so in order
for the dispatch of INITIALIZE-INSTANCE to work, the instance must
already have the unique number of its class stored in it.
Moved definition of *t* to before discriminating function.

During generic function dispatch, we currently consider every non-heap
instance to be an instance of the class T.  So if we are asked to find
the instance unique class number, and we are given something other
than a heap instance, we must be able to access the class T and return
its unique class number.
Use unique class numbers in generic function dispatch.

This version of the generic function dispatch is a step on the way
toward the final goal of fast generic function dispatch.

So far, we have implemented the CALL HISTORY which is basically a
cache of what COMPUTE-APPLICABLE-METHODS-USING-CLASSES returns,
together with the resulting effective method.  The discriminating
function consults the cache before calling the machinery of computing
applicable methods and effective methods.