0.3.0

Added:

• implementations of ToDXR and FromDXR for Box<T> (making owned recursive types possible)
• support for deriving ToDXR for structs containing &T references as fields (making
  reference-based recursive types possible)

Changed:

• merged code from the dxr_shared crate into the main dxr crate
• removed support for non-standard i8 and nil values from default features
• split server feature into generic server functionality and axum support
• use anyhow to simplify error handling in client and server implementations
• conversion from FaultResponse to Fault can fail, so implement a fallible
  TryFrom instead of a panicking From

Internal changes:

• added trybuild based pass/fail tests for derive macros