Add max_member_weight to sum_of_members selector semantics (and change to oneof).

[jonathan-dilorenzo]

As per #396. I found the phrasing a bit awkward. Maybe y'all have better ideas?

[jonathan-dilorenzo]

Fixed! Thanks for comments!

[antoninbas]

@jonathan-dilorenzo will you need help with generated files?

[jonathan-dilorenzo]

Yes please! I gave it another shot, but I have some permission issue when I try it :'(.

[antoninbas]

@jonathan-dilorenzo done, I updated your branch

[antoninbas]

@smolkaj any comment on this? otherwise I'll go ahead and merge it

[smolkaj]

Looks good to me.