Stream: fix doc bug
Stream: remove extraneous Streamable module def

Should have been done with 116c39bd60061a5dfbeaa81460376630b9113a44.
remove extraneous streamable type definitions

Should have been done with 5827bcf946228fcb9c4320cfed4c10c094dff278.
Streamable: rename stream to to_stream

This follows Core style.
Lines: implement group2 transform
Lines: define Error sub-module
Stream: minor doc fix to section headers