ce: for items, Python-spelled, in every builder

[simontreanor]

Closes #103 (the for item; the diagnostic and docs-table halves shipped in #112; custom operations are recorded as turned down in ROADMAP item 15).

for target in source: in every builder

The Python spelling, consistent with match e: / case. for is a keyword only in item position inside CE braces (Pyfun has no loop statement elsewhere). The source is a List a or a Seq a (an unresolved source is taken to be a List), the target any irrefutable pattern (for (a, b) in pairs: unpacks in the Python header), and the body is a nested level of the same block: anything but return, which is rejected with a message saying the block's value comes after the loop.

• seq lowers to Python's for statement, the idiomatic generator that yield! (List.map f xs) was standing in for.
• async loops with awaits inside the async def.
• result / option: a failed let! inside the loop short-circuits the whole block; the items after the loop follow the for statement.
• User builders get F#'s For: B.for_ e (fun x -> body), combined with what follows like a yield. The issue's Html { } DSL now writes for r in rows: yield tr r.

let squares = seq {
  for x in [1, 2, 3]:
    let y = x * x
    yield y
  yield 100
}

def _pf_fn0():
    for x in [1, 2, 3]:
        y = x * x
        yield y
    yield 100

One layout rule

Braces carry no layout tokens, so the body is delimited by columns like every other item, with one Python-shaped rule: a body that begins on the same line as the : is exactly one item (for x in xs: yield x), and a longer body goes on indented lines. The pretty-printer keeps one-line CEs one-line and lays a block out on lines only when a for body has several items, so parse ∘ print stays idempotent (roundtrip tests added for both shapes, including nested loops). Inside CE braces for also stops an application, so yield a for c in cs: yield c is two items.

Elsewhere

CeItem::For is walked by the capture census, the fold pass's free-variable and binder collectors, the active-pattern arity scan, and the LSP resolver (the target is a nested scope, so go-to-definition and rename see it). DESIGN.md §8.1 gets the row and a paragraph; the internals table, lesson 13 (a for section in seq) and lesson 20 (for_ in the protocol) are updated; ROADMAP item 15 is closed.

Tests

Typecheck (every builder, the Seq string element type of a tuple target, return-in-loop and non-collection-source rejections), roundtrip (one-line and multi-line shapes), a parser unit test for the near-miss diagnostic, and one e2e program running all four builders. cargo test, cargo clippy --all-targets, cargo fmt --check and docs/verify_lessons.py are clean.