Blit_fill copies between iobufs and advances dst but does not advance src.
val blit : src:([> Core__.Import.read ], no_seek) t ‑> src_pos:int ‑> dst:([> Core__.Import.write ], seek) t ‑> len:int ‑> unitval blito : src:([> Core__.Import.read ], no_seek) t ‑> ?src_pos:int ‑> ?src_len:int ‑> dst:([> Core__.Import.write ], seek) t ‑> unit ‑> unitval unsafe_blit : src:([> Core__.Import.read ], no_seek) t ‑> src_pos:int ‑> dst:([> Core__.Import.write ], seek) t ‑> len:int ‑> unit