v0.4.0

• Breaking: Remove the --yarpc flag. Install the ThriftRW YARPC plugin
  from go.uber.org/yarpc/encoding/thrift/thriftrw-plugin-yarpc and use
  --plugin=yarpc instead.
• Breaking: The compile API now exposes annotations made while
  referencing native Thrift types. This changes the TypeSpecs for primitive
  types from values to types.
• The compile API now also exposes annotations for typedef declarations.
• Generate args structs and helpers for oneway functions.
• Expose whether a function is oneway to plugins.
• Expose the version of the library under go.uber.org/thriftrw/version.Version.
• Generated code will test for version compatibility with the current version
  of ThriftRW during initialization.