Add FromZval derive macro

[davidcole1340]

Ability to derive FromZval (and potentially IntoZval) using a derive macro. Could be used in two situations:

• Enums. Data type would be mixed, and would act as a mixed type parameter. Example:

#[derive(FromZval)]
pub enum IntOrBool {
    Integer(u32),
    Boolean(bool),
}

// translates to

impl FromZval<'_> for IntOrBool {
    const TYPE: DataType = DataType::Mixed;
    fn from_zval(zval: &'_ Zval) -> Option<Self> {
        if let Some(integer) = u32::from_zval(zval) {
            Some(integer)
        } else if let Some(boolean) = bool::from_zval(zval) {
            Some(boolean)
        } else {
            None
        }
    }
}

• Structs. Data type would be object, and the struct fields would be read from the objects properties. Primarily for passing stdClass into Rust. Example:

#[derive(FromZval)]
pub struct ExampleStdClass {
    a: i32,
    b: String,
    c: bool,
}

// translates to

impl FromZval<'_> for ExampleStdClass {
    const TYPE: DataType = DataType::Object(None);

    fn from_zval(zval: &'_ Zval) -> Option<Self> {
        let obj = zval.object()?;

        Some(Self {
            a: obj.get_property("a").ok()?,
            b: obj.get_property("b").ok()?,
            c: obj.get_property("c").ok()?,
        })
    }
}

[vodik]

I like it. It should be paired with a IntoZval as well.

Something worth keeping an eye on is PHP 8.1 enums. I don't think this should conflict through: a reasonable approach is probably to either add a new #[php_enum] attribute (or make php_class smarter - idk how its implemented under the hood) and make sure that its mutually exclusive from these derive macros.