Add support for parsing URL encoded form arrays of structs

[adam-fowler]

We should be able to encode and decode arrays of structs eg

struct Person {
    let name: String
    let age: Int
}
struct People {
    let p: [Person]
}

should be represented as

p[0][name]=John&p[0][age]=23&p[1][name]=Jane&p[1][age]=67

[connor-ricks]

Whatever you land it, it might be relevant to allow overriding of the encoding/decoding strategy for arrays specifically. I don't think there is an official standard for handling arrays in query parameters, but there are a few "common" implementations, so it might be worth having a "strategy" style enum.