NGC: make savedata format all msgpack, not a mix of msgpack and custom binary stuff

[iphydf]

Steps to make it backwards compatible (for a while):

1. Change "save groups" to only write msgpack objects, no custom binary formats. Also merge state_bin and state_values.
2. Change gc_load_unpack_group to check the array size: 6 means new format, 7 means old format.
3. Add a set of "load" functions to load the new format.
4. Release 0.2.19.
5. Do stuff for 0.2.20.
6. Release 0.2.20.
7. Delete old format support; do stuff for 0.2.21.
8. Release 0.2.21.

This way we'll have some backwards compatibility for 2 versions and then ditch it. Hopefully people have upgraded by then. If not, they may lose their groups.

[Green-Sky]

After we drop the old compat, we could just fail to load the file, and error that the user should load the profile with 0.2.19 to convert the file.
#1944 is probably needed for failing.