Support positional destructuring of structs

[timfennis]

Structs (#197) can currently only be read field-by-field. Positional destructuring in declaration order — mirroring the positional constructor — would allow:

struct Foo { a: Int, b: Int }
let f = Foo(1, 2);
let x, y = f;
assert_eq(x, 1);
assert_eq(y, 2);

Today both paths reject this cleanly:

• statically known type: error[resolver]: Invalid unpacking of Foo
• dynamically typed receiver: error[vm]: cannot unpack: expected a list, tuple, or string

Unpacking is currently defined only for lists, tuples, and strings.

Design questions

• Should struct unpacking also work in for loops and other destructuring positions?
• Is partial unpacking (let x, y = f on a 3-field struct) an error, like it presumably is for tuples?
• Should the analyser require the arity to match the field count at compile time when the struct type is known?

🤖