Derive `Deserialize` for `Level`

[max-ishere]

It would be nice to have this trait on the log crate (even behind a feature flag) to allow applications to use serde to parse the log levels.

The example use case would be a log level contained in a config file that is parsed with serde.

While looking up enum serialization I found this: Serialization of an enum variant with serde which makes it look like a simple derive macro would be enough to add to the code.

[max-ishere]

Oops, my bad, got confused by the compiler error and didnt see that it was behind a feature flag