Discussion for proposal 2019-001 (Correction to the PRIM_IO signature)

[JohnReppy]

This issue is for discussion of proposal 2019-001 (Correction to the PRIM_IO signature).

[MatthewFluet]

Seems reasonable; MLton currently just performs the Position.int to int conversion in the avail function: https://github.com/MLton/mlton/blob/master/basis-library/posix/io.sml#L319