API exposes private type

[bbjubjub2494]

Description

A bunch of methods in Using have OpenFile[T] as their return type, which is private.

How to reproduce

View the sbt.io.Using class in scaladoc and look at e.g. fileInputStream.

Expected behaviour

The type should be more descriptive. It took me a while to figure out that it was a subtype of Using[File, T], so I reckon changing the exposed type could improve UX. I don't know if there's a particular reason.

This would break binary compatibility.