util.stanza: Allow U+7F
Allowed by XML despite arguably being a control character.
Drops the part of the range meant to rule out octets invalid in UTF-8
(\247 starts a 4-byte sequence), since UTF-8 correctness is validated by
util.encodings.utf8.valid().
Structure of a stanza:
stanza {
--- properties ---
tags -- array of tags
--- static methods ---
iq(attrs) --
--- read-only methods ---
reply -- return new stanza with attributes of current stanza
child_with_name(string name) -- return the first child of the current tag with the matching name
--- write methods ---
tag(name, sttrs) -- create a new child of the current tag, and set the child as current
up() -- move to the parent of the current tag
text(string) -- append a new text node to the current tag
}