Fix tag_expression usage of frozen_children which was tag objects.
Fix tag_expression usage of frozen_children which was tag objects.

At some point, I changed tag_export.flat_dict from using strings as
keys to using actual Tag objects as keys. The tag expression handler
never got updated, so frozen_children[tagname] raised KeyError as
it was looking for a string.
I considered using tag_expression_tree.map to convert all the tree
tokens into tag objects, but when we render the tree back into text
it will say "Tag:name" instead of just "name" throughout the whole
expression, and I don't want to deal with converting those back.

voussoir/etiquette,voussoir/etiquette,voussoir/etiquette