Gff: add (pure) function `item_to_string`
Fasta: put type raw_item at the toplevel (#91)
Fasta: rename `Result` and `_exn` functions (#91)
Fasta: add functions `in_channel*raw_item*` (#91)
Fasta: add {char,int}_seq_raw_item_to_string (#91)
Fasta: fix missing documentation