Sgr: remove use of old Lines module

Slight API change: now Sgr.of_channel and Sgr.of_file raise Failure
instead of Lines.Error, but we need to re-write Sgr anyway to follow
new guidelines.
Transcripts: remove use of old Lines module

Slight API change: now of_composite_channel and of_composite_file may
raise Failure instead of Lines.Error, but we need to re-write
Transcripts anyway to follow new guidelines.
Internal_pervasives.Lines: delete module

This module is a remnant from the Sesame library that Biocaml used to
depend on. (Note this is unrelated to the new Lines module.)
Internal_pervasives.flip: delete

Already available in Core as Fn.flip.
define Stream.unfold

Mimics Batteries' Enum.unfold.