Populate GuildID on GuildMembers (similar to GuildMember)

[bamo]

Currently, GuildMember sets member.GuildID, but GuildMembers does not do the same, leaving it empty.

This results in confusing behavior where, if the caller does not set GuildID themselves, calls like member.AvatarURL("") can return invalid URLs.

I don't think there should be any adverse effect to doing so - any performance implications of setting one field on each member struct should be negligible when compared with the JSON unmarshal that's already happening there.