Added support for newtype structs and serde flatten

[ohadravid]

This PR adds support for two related things: newtype-structs and serde flatten.

Example for newtype-structs:

#[derive(Deserialize, Debug)]
struct Win32_OperatingSystem(pub HashMap<String, Variant>);

let system = wmi_con.query::<Win32_OperatingSystem>().unwrap();

Example for serde-flatten:

// Using `raw_query`:

#[derive(Deserialize, Debug)]
struct Win32_OperatingSystem {
    Caption: String,
    Name: String,

    #[serde(flatten)]
    extra: HashMap<String, Variant>,
}

let system: Vec<Win32_OperatingSystem> = wmi_con
    .raw_query("SELECT * FROM Win32_OperatingSystem")
    .unwrap();

// Or, using a newtype wrapper:

#[derive(Deserialize, Debug)]
#[serde(rename = "Win32_OperatingSystem")]
struct Win32_OperatingSystemWrapper(pub Win32_OperatingSystem);
let system = wmi_con.query::<Win32_OperatingSystemWrapper>().unwrap();

(Regular .query cannot work because of serde-rs/serde#1346)

Fixes #109