Unifying the variable naming of sequences

[stefan-aws]

Currently, we use all sorts of variable names (s, s', a, b, xs, xx ...) for sequences. I propose we stick to xs, ys, zs, ... in the standard library, where possible. The main reasons for this change are:

• Consistency
• Used in e.g. Ocaml (https://v2.ocaml.org/api/Seq.html) and Haskell (https://hackage.haskell.org/package/containers-0.6.6/docs/Data-Sequence.html)
• In https://github.com/dafny-lang/libraries/pull/66, sequences are mixed with sets. I propose we reserve s for sets, as is practise e.g. in OCaml (https://ocaml.org/docs/sets)

I also change some of the existing comments, and add new ones.

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