Require explicit struct types in literals and destructuring

[someone235]

Fixes #157

Struct literals and destructuring patterns now carry an explicit struct name. Struct values are checked nominally instead of being inferred from matching field layouts, making same-shaped
structs unambiguous across assignments, state readers, template validation, and struct arrays.

Language Behavior Changes

• Struct literals must include their declared type:

Receipt receipt = Receipt {
    orderId: orderId,
    buyer: buyer,
    amount: amount
};

Anonymous literals such as { orderId: orderId, ... } are no longer supported.

• Struct destructuring must include the expected type:

State {
    count: int previousCount,
    owner: pubkey previousOwner
} = readInputState(1);

• Typed destructuring disambiguates structs with identical layouts:

AState {
    value: int previousValue
} = readInputStateWithTemplate(
    1,
    prefixLength,
    suffixLength,
    templateHash
);

• Template-output builtins accept inline typed struct literals:

validateOutputStateWithTemplate(
    0,
    Receipt {
        orderId: orderId,
        buyer: buyer,
        amount: amount
    },
    templatePrefix,
    templateSuffix,
    templateHash
);

• Struct assignments, arguments, and array elements must use the expected nominal struct type. A different struct with the same fields is no longer accepted based solely on its layout.