output: Make groups contain their subitems, and get rid of spv_item.

output: Make groups contain their subitems, and get rid of spv_item.

Until now, the output subsystem has had two different ideas for output
items: struct output_item, which is the primary representation, and
struct spv_item, which represents an output item read from an .spv file.
The biggest difference, until now, has been that spv_item contains its
children, whereas output_item bracketed children inside open/close pairs.
This commit unifies them under output_item, making output_item adopt the
container abstraction.