[patch[ Add explanations to sbitmap, bitmap, and sparseset

[Steven Bosscher]

Hello,

Add some explanations because I see a lot of places where the choice
of data structure is less than optimal.

Also some functional changes: sbitmap with popcounts are only used in
ebitmap, and they penalize the "normal" case without popcount a lot.

Bootstrapped&tested on powepc64-unknown-linux-gnu. OK?

Ciao!
Steven
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