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Add more bad input tests for ARITY to prepare for some refactoring of proc_name_arg().

Rewrite proc_name_arg() in terms of validate_argument().
This is a step toward removing the side effect of modifying the arguments list.
Unfortunately, this required making IS_VALID_ARGUMENT accept a non-const NODE* because numberp() modifies its input.

Fix Bug #612; COPYDEF of procedure defined with DEFINE now has the correct title line.

Change mustDerefStringnode to mustGcrefStringnode in numberp().
A ref and a deref are not necessary. A simple gcref suffices.
Unfortunately, the flag is still needed because numberp's argument may not have any references.

Add a missing line break

Refactoring: Change lbutlast to use CAppendableList.
This reduces code duplication.

Rework char_arg to take the argument instead of an argument list.
Also, rename it to validate_char_argument since it follows the new conventions of the validate* functions.

Refactoring: Change the signature of char_arg() to return a wchar_t instead of NODE*.
This is what all of the callers want, anyway.

This change makes it clear that char_arg() is responsible for freeing whatever it allocates.

Rework char_arg() so that it doesn't modify the input list.
This is part of a wider effort to simplify the code base by removing side effects.

Add additional tests for ASCII to prepare for refactoring of char_arg().