is valid if the vector is being allocated in the heap, but not if it is being
allocated on the stack. You could remove this optimization, but that makes
the heap-allocated case somewhat slower...)
+
+To do this, extend ALLOCATE-VECTOR with ALLOW-JUNK argument, and when
+stack allocating don't zero if it is true -- and probably ALLOW-JUNK iff
+the vector is a specialized one (cannot have pointers.)
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#28
a. Accessing raw slots in structure instances is more inefficient than