DEFINE-DEPRECATED-{FUNCTION,VARIABLE} improvements
* At runtime, deprecated variables in the :FINAL state behave like
functions in that state: instead of an "undefined variable" error, a
DEPRECATION-ERROR is signaled.
* GLOBAL-SYMBOL-VALUE checks for deprecated variables at compile-time
like SYMBOL-MACRO already did.
* DEFINE-DEPRECATED-FUNCTION calls DEPRECATED-FUNCTION correctly and
checks argument types.
* Some technicalities:
* Moved PRINT-SYMBOL-WITH-PREFIX to early-extensions.lisp to make it
available in the deprecation machinery.
* DEFINE-DEPRECATED-VARIABLE checks argument types.
* Added tests.