Destructuring patterns

Anywhere Rust accepts a binding, it accepts a pattern. That is one rule with a lot of reach: `let`, function parameters, `for` loops, `match` arms and `if let` all use the same syntax.

Destructuring keeps names close to the data they describe, and it avoids a trail of `.0`, `.1` and `.2` accesses that nobody can read later.

The `..` pattern skips the fields you do not care about, and `_` ignores a single value without binding it.

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