Proc macro development utilities

Features

| Feature | Description | | ------- | ----------- | | default | ["derive"] | | attr_parse | Attribute parsing utilities, attr_parse. | | derive | Enable derive macros. Currently requires the attr_parse feature to do anything. | | full | Enable syn/full. If attr_parse is enabled, it'll implement the traits for types that require syn/full. | | nightly | Enable some nightly Rust optimisations during macro execution only, has no effect on generated code. |