It's really easy to accidentally pass an uninitialized string as dst into
the copy function, which will result in an free() call to an arbitrary
pointer. Maybe it's a better idea to not deinit the dst string before
copying? The documentation at least makes it more clear and the new
apfl_string_blank() function makes it easy to create an empty string.
With the varargs approach that was used before, it was very easy to add a
list item of the wrong type, which would (hopefully) result in an assertion
violation, because va_arg() then read some senseless data.
