Improve accuracy of wml/is_tag.

By checking that the input has length 2.
Enable wml lib functions that expect a cfg to take a tag instead.
Conformed wml/{tags,cfgs}_equal to docs.

For the former, remembered to check that the tags have the same name.

For the latter, remembered to actually implement it at all.
Improve resilience of wml/to_{tag,cfg} and make them public.
Publish [remove_object] documentation.