Support protobuf NEW operators

[apstndb]

Part of #89

This PR adds NEW operators for Protocol Buffers.

https://cloud.google.com/spanner/docs/reference/standard-sql/operators#new_operator

NEW protocol_buffer {...}: Creates a protocol buffer using a map constructor.

NEW protocol_buffer {
  field_name: literal_or_expression
  field_name { ... }
  repeated_field_name: [literal_or_expression, ... ]
}

NEW protocol_buffer (...): Creates a protocol buffer using a parenthesized list of arguments.

NEW protocol_buffer(field [AS alias], ...field [AS alias])

[apstndb]

Rebase to use ast.NamedType.

[apstndb]

Updated to use helper functions #120

[apstndb]

Merge to use parseCommaSeparatedList().

[apstndb]

Merge to include ARRAY fix