fix: hide file::reader

Fixes #202. Apparently this can fail compilation on 1.44. The content
method on FileReader exposes a non-public item through impl
Iterator<Item = (..., non_pub_here)> but still the error is about
lifetime capture.

unixfs/src/file.rs

@@ -8,7 +8,7 @@ use std::borrow::Cow;
 use std::fmt;
 
 /// Low level UnixFS file descriptor reader support.
-pub mod reader;
+mod reader;
 
 /// Higher level API for visiting the file tree.
 pub mod visit;