Remove the `pack!`, `pack_err!`, `pack_structural!`, and
`pack_err_structural!` macros, replacing all pipeline type definitions
with `#[derive(Grouped)]` and `#[derive(Grouped, Wrap)]` attributes.
This changes the generated struct shape from named-field structs with an
`inner` field to tuple structs accessed via `.0`, and removes the
auto-generated `name` and `info` fields from error types.

The `dispatcher!` macro no longer requires a `CMD*` dispatcher type
argument; the dispatcher struct is now generated internally as
`__Dispatcher{Pascal}`. The `Node` type, `node!` macro, and
`Dispatcher::node()` / `clone_dispatcher()` methods are removed.