update AST-to-HIR lowering examples for conditionals and loops

- `for` loops now use two `match`es for all of their bindings. I'm not
sure this is the most helpful way of conveying that, but it's about as
informative as before while staying brief.

- `while let` and `if let` don't use `match`; they use `let` expressions
in their conditions. Since `if let` no longer has significantly
different desugaring and having a whole bullet point for `while` would
feel redundant with `for`, I've removed those examples.

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