CommandBuffer insert/remove ordering is a footgun

[Ralith]

CommandBuffer allows deferred execution of component insertion and removal. At present, it executes all insertions first, then all removals. This might not reflect the user's intention after calling e.g. cmd.remove_one::<T>(); cmd.insert_one::<T>(x);, which could easily happen if one command buffer is reused across widely separated code, a pattern which is normally desirable to improve resource reuse.