refactor(sse): DataStream: more intuitive colocation and interface

[kanarus]

DataStream, the core type for streaming response, before lived in typed module unintuitively, and its interface is so too due to some confusion around the type of stream item ( T v.s. Result<T> ) and the conversion ( Into<String> ).

This PR migrates it into sse::{DataStream, Data}:

• Place DataStream in new module named sse. This is intuitive.
• And refactor it to have more clear interface to asynchronously send T items.
• Use Data trait instead of Into<String> for data conversion. This makes the context clear.

In addition, this PR eliminates a double-boxing of previous SSE implementation.