Better use of `Maybe`

[maciejhirsz]

• Extracted the Maybe trait into its own module
• Setters for optional parameters now accept a generic Maybe<T> implementation instead of just T.
• Added generic impl<T> Maybe<Option<T>> for T and impl<T> Maybe<T> for Option<T>.

This makes optional parameter handling really nice without any bloat to the API surface.

Taking an Option as a parameter:

#[component(
    // Make `name` an optional parameter, defaults to `"Kobold"`
    name?: "Kobold",
    // Make `age` an optional parameter, use the `Default` value
    age?,
)]
fn Greeter<'a>(name: &'a str, age: Option<u32>) -> impl View + 'a {
    let age = age.map(|age| view!(", you are "{ age }" years old"));

    view! {
        <p> "Hello "{ name }{ age }
    }
}

view! {
    // Hello Kobold
    <Greeter />
    // Hello Alice
    <Greeter name="Alice" />
    // Hello Bob, you are 42 years old
    <Greeter name="Bob" age={42} />
}

Using Option to set an optional parameter:

#[component(code?: 200)]
fn StatusCode(code: u32) -> impl View {
    view! {
        <p> "Status code was "{ code }
    }
}

view! {
    // Status code was 200
    <StatusCode />
    // Status code was 404
    <StatusCode code={404} />

    // Status code was 200
    <StatusCode code={None} />
    // Status code was 500
    <StatusCode code={Some(500)} />
}