constructors that return result<>

[OmarTawfik]

Today, the following syntax is illegal, since constructors have no return types:

resource blob {
    constructor(data: list<u8>) -> result<blob, error-type>;
}

However, this is a perfectly valid use case for languages like JS and C++, as result<> return type are lowered into blob directly, using exceptions to propagate error-type values if they occur. This is what happens already with normal methods that return result<>.

Thoughts? I wonder if it is possible to enable this use case. Without it, APIs are forced to use alternative static methods like create: static func(), which is not standard, and hurts readability/discoverability.

Moved from bytecodealliance/wit-bindgen#1114