Key-value serializer adapters

[KodrAus]

This PR does two things:

• Exposes the Value::downcast_ref method that was missed in Replace value internals with external crate #423
• Adds adapters so that Sources can be treated like a map or sequence for formatting or serialization

This means Log implementors can now write something like:

use log::kv::source;#[derive(Serialize)]pubstructMyRecordAsMap<'a>{msg:&'astr,#[serde(flatten)]#[serde(with = "source::as_map")]kvs:&'adynSource,}

to define a type that can be serialized using serde as a map with key values flattened.

[passcod]

Would be nice to add that code sample to the kv docs as it's not super obvious how to use it otherwise