Impossible to decode rmp-serde Ext type even with own trait implemented for rmp_serde::decode::Deserializer

There is no way (or it would be very hacky) to read the Ext meta data from the inner `rd` reader (the `get_ref` method only exposes the internal implementation of `rd` but not the `rd` itself).

My approach was to implement my own trait for rmp_serde::decode::Deserializer like this:
```rust
impl<'de, 'a, R> MyDeserializer<'de> for &'a mut rmp_serde::decode::Deserializer
where R: rmp_serde::decode::Read<'de>
{
fn deserialize_foo>(self, visitor: V) -> Result {
self.deserialize_any(visitor).or_else(|err| {
if let rmp_serde::decode::Error::TypeMismatch(marker) = err {
let meta: ExtMeta = self.somehow_get_ext_meta()?;
// do something with meta, .e.g. call self.deserialize_any again if type matches
return Ok(value);
}
Err(err)
})
}
}
```
But, there is no way to read the ExtMeta from the internal `rd` reader of Deserializer. I think the ExtMeta could be easily returned from this place:
https://github.com/3Hren/msgpack-rust/blob/master/rmp-serde/src/decode.rs#L347
like this:
```rust
marker => {
let size = match marker {
Marker::FixExt1 => 1,
Marker::FixExt2 => 2,
Marker::FixExt4 => 4,
Marker::FixExt8 => 8,
Marker::FixExt16 => 16,
Marker::Ext8 => read_u8(&mut self.rd)? as u32,
Marker::Ext16 => read_u16(&mut self.rd)? as u32,
Marker::Ext32 => read_u32(&mut self.rd)?,
marker => return Err(Error::TypeMismatch(marker)),
};
let ty = rmp::decode::read_data_i8(&mut self.rd)?;
let meta = rmp::decode::ExtMeta {
typeid: ty,
size: size,
};
Err(Error::ExtensionFound(meta)) // Would require an ExtensionFound(ExtMeta) variant in Error enum
}
```

Adding a method to get access to the Deserializer `self.rd` would also solve the problem anyway.