Defines a top level (global) lexical VAR with initial value VAL,
which is assigned unconditionally as with DEFPARAMETER. If a DOC
string is provided, it is attached to both the name |VAR| and the
name *STORAGE-FOR-DEFLEX-VAR-|VAR|* as a documentation string of
kind 'VARIABLE. The new VAR will have lexical scope and thus may
be shadowed by LET bindings without affecting its global value.