0.2.0

This version contains some fixes that required changes in public APIs (listed below), so this
is a semver-incompatible release.

Added:

• convenience methods for checking the type of DxrError and extracting the inner error value
• more client-server roundtrip tests (for ~80% test coverage across all three crates)
• support for implementing / requesting graceful server shutdown
• snapshot tests for some Debug implementations
• feature hints for the generated documentation on docs.rs

Changed:

• moved method handler synchronization from the server to the method handlers themselves to
  improve concurrency, throughput and latency, especially for methods that do not modify internal
  or global state
• changed MethodResponse to match the XML-RPC spec (<params> can be missing)
• changed method signatures on client and server implementations to adapt to optional return values
• renamed DxrError::ReturnMismatch to DxrError::ParameterMismatch to better reflect what it
  actually means (unexpected number of parameters, whether they are method call arguments or
  return values)