Nested streamed aggregated data types

[BridgeAR]

The spec does not specify if the newly added streamed aggregated data types may be nested or not. It is of course possible to implement this in a parser but it complicates the logic for "event based languages" such as JavaScript since it's required to keep lots of state around to keep the parser fast. Limiting the streamed aggregated data type to only be allowed in the top level would simplify that.