Line,Lines: improve documentation
Sam: add `missing in_channel_to_*` functions
Sam: improve documentation and API
Wig: clean-up error types
Wig: rename `t` to `item` (homogenization)
Wig: add `Error.to_bed_graph`
Wig: add and document `in_channel_to_*` functions
Bed: add type `Error.t`
Bed: add `in_channel_to_item_stream{,_exn}`