Extend wml/to_cfg with optional tag name checking.
Add internal utils/tblorudt function.
Fix wml/to_cfg; I had neglected to provide a filter when calling match.
Fix wml/{remove_subtags,remove_children,erase_subtags}.

Adjust iterator index after deleting subtags.
Enable wml/{get,remove}_{subtags,children} to return reversed lists.

Add parameters to specify this.

Should be more generally more efficient, esp. with
remove_{subtags,children}, because those would otherwise flip the list
anyway.
Initial implementation of modifications lib.