Add decode_any through Visitor

[udoprog]

This is the last missing API piece I can think of:

A thing decoding might want to ask the decoder if it knows what type is being decoded.

Previously this was handled through Decoder::type_hint, but this has a runtime penalty: The returned TypeHint needs to be matched over.

Instead for this we adopt a modified Visitor pattern from serde, the twist being that the visitor can produce its own sub-visitors: type String, type Bytes and type Number. This makes visitors more composable since it's a more granular set of traits you need to implement and use instead of one big Visitor trait.

The #[musli::visitor] attribute also has to be added to allow for API evolution.

This is what its use looks like in musli-value:

#[musli::visitor]
impl<'de, M, E> Visitor<'de> for AnyVisitor<M, E>
where
    M: Mode,
    E: Error,
{
    type Ok = Value;
    type Error = E;

    type String = StringVisitor<E>;
    type Bytes = BytesVisitor<E>;
    type Number = ValueNumberVisitor<E>;

    /* ... */
}