AsRef restricted impls

[MegaBluejay]

Currently there are 2 options for AsRef derives:

• the default behaviour, where only implementations for field types are generated
• [as_ref(forward)], which generates a generic impl for all types the field can be converted to

It would be useful to be able to list types for implementation explicitly, like with From

For example

#[derive(AsRef)]
struct Test {
    #[as_ref(str)]
    str_field: String,
    #[as_ref([u8])]
    u8_field: Vec<u8>,
}

This wouldn't work with [as_ref(forward)], since we'd have conflicting implementations for AsRef<[u8]>.