doctool/Gjs: some more fixes for structs and unions

Stop pretending we have fields on unions (only methods are
supported).
Add minimal support (ie, don't crash) to nested structures,
that due to how ast works have namespace None (and the
transformers hard-depend on that). Uncovered by GLib's
GDoubleIEEE754, before I removed union fields.
For some reason, RegressTestStructE (anonymous union) has a
completely different behavior and generates a weird name,
while RegressLikeGnomeKeyringSchema (array of unnamed structs)
becomes array(gpointer).
Bah, one should have methods anyway...