stdlib: complete the Seq module

[simontreanor]

Dogfooding finding #5, PR 2 of 5 (see #34). Follows #42 (List) and #43 (member docs).

Seq carried 7 members against List's 51, which made the lazy half of the library the one you left for a list plus a conversion. This adds 22, and the single-pass caveat turned out to be the organising idea rather than a footnote — every member's hover line now says which half it is in.

Lazy — a new sequence, nothing pulled yet: drop takeWhile dropWhile concat flatten collect zip indexed distinct pairwise init initInfinite unfold. Most route to Python's own lazy machinery (itertools.islice/takewhile/dropwhile/chain/pairwise/count), so a chain of them stays lazy. distinct and unfold are emitted generators, since they carry state while staying lazy.

Consuming — a Python iterator is single-pass, so what these pull is gone: head isEmpty len exists forall contains sum iter find. Several short-circuit, consuming only as far as the answer.

initInfinite and unfold have no list behind them, which is the reason the lazy module exists:

Seq.initInfinite (fun i -> i * 2) |> Seq.take 5 |> Seq.toList   # [0, 2, 4, 6, 8]
Seq.unfold (fun s -> if s > 100 then None else Some (s, s * 2)) 1 |> Seq.take 4   # [1, 2, 4, 8]
Seq.find (fun x -> x > 6) (Seq.initInfinite (fun i -> i * 2))   # Some(8)

Two end-to-end tests build endless sequences deliberately. Neither terminates if a member that claims to be lazy quietly forces its input, so they check the laziness itself rather than asserting on emitted text.

Naming follows List, not F#: drop rather than skip, and concat appends while flatten flattens — the same divergence already taken for List, kept consistent so the two modules read the same way.

Documentation carries the cost, which matters more here than for List: that Seq.len walks the whole sequence and never returns on an endless one is exactly what a reader needs before calling it, and that is now in hover rather than in DESIGN prose.

Tests: 7 typecheck cases (lazy members stay in Seq, Seq and List members do not mix, unfold must answer Option, iter carries its effect), 3 lowering assertions (routes to the lazy machinery, distinct yields rather than building a list, unfold's loop), and 3 end-to-end programs including the two endless ones. Full suite, clippy and fmt clean.