Added test_user_command in test_tcl.

It tests the convertion Tcl values to Python values when Tcl calls a command
implemented on Python. Currently all values are passed as strings.
Added test_user_command in test_tcl.

It tests the convertion Tcl values to Python values when Tcl calls a command
implemented on Python. Currently all values are passed as strings.
asyncio: Fix open_connection() documentation, writer is a StreamWriter
asyncio doc: move stream classes to their own class
asyncio doc: link create_connection() to open_connection() and create_server()
to start_server()

Rename also the "Network functions" section to "Stream functions" and move it to the Stream section.
asyncio doc: Move streams to their own dedicated page
asyncio doc: add an client example using streams