feat!: normalize empty lists and direct Read output

[wackywendell]

Description

This PR makes empty lists explicit across the text format and uses compact => as the canonical spelling for a named Read with Direct output.

For example, both of these inputs describe an empty named Read schema:

Read[my.table => _]
Read[my.table +> _]

Formatting writes the first form. A Read with an explicit Emit remains explicit:

Read[my.table +> a:i32, b:string |> $1, $0]

Part of #34.

Changes

• (Breaking!) Requires _ for empty extension and addendum arguments, extension output
  columns, named and virtual Read schemas, and Aggregate output.
  • Reject blank required lists such as Read[my.table +> ], Read:Virtual[() => ], and Aggregate[_ => ].
• Canonicalize named Read Direct output as => schema, including verbose
  formatting and protobuf input that explicitly contains Direct.
• Continue accepting Read[table +> schema] as Direct input, but format it
  with =>.
  • The explicit Read[table +> schema |> mapping] would be preserved.