Print macro modified macro keys as characters not integers

Print macro modified macro keys as characters not integers

* lisp/macros.el (macros--insert-vector-macro): Pass all elements to
'prin1-char', not just those that satisfy characterp (because characters
which have modifier bits set wouldn't qualify otherwise).
'prin1-char' will return nil if it can't handle the argument (e.g.,
for symbols representing function keys).