use unbound fluids instead of `void' sentinel value

* module/language/elisp/compile-tree-il.scm (reference-with-check)
  (compile-without-void-checks, want-void-check?): Remove.

  (compile-function, compile-pair): Use `reference-variable' instead of
  `reference-with-check'.

  (compile-defvar): Only set `sym' if `sym' is not bound to a bound
  fluid, rather than requiring that its value be `void'.

  (process-options!): Remove `#:disable-void-check' option handling.

* module/language/elisp/runtime.scm (void)
  (reference-variable-with-check): Remove.

  (ensure-fluid!): Use an undefined fluid as the initial value for
  global variables.

* module/language/elisp/runtime/function-slot.scm (without-void-checks):
  Don't import or re-export.

* module/language/elisp/runtime/macros.scm (prog1, cond, or, dolist):
  Don't use `without-void-checks'.

* module/language/elisp/runtime/subrs.scm (symbol-value)
  (symbol-function, apply): Use `reference-variable' instead of
  `reference-variable-with-check'.

  (makunbound, fmakunbound, boundp, fboundp): Unset the variable's fluid
  (or the variable itself, if it isn't bound to a fluid).

* test-suite/tests/elisp-compiler.test ("Variable
  Setting/Referencing")["disabled void check (all)", "disabled void
  check (symbol list)", "without-void-checks"]: Remove.

module/language/elisp/runtime/function-slot.scm

@@ -48,7 +48,6 @@
                  (compile-let* . let*)
                  (compile-lexical-let* . lexical-let*)
                  (compile-flet* . flet*)
-                 (compile-without-void-checks . without-void-checks)
                  (compile-with-always-lexical . with-always-lexical)
                  (compile-guile-ref . guile-ref)
                  (compile-guile-primitive . guile-primitive)
@@ -71,7 +70,6 @@
                let*
                lexical-let*
                flet*
-               without-void-checks
                with-always-lexical
                guile-ref
                guile-primitive