Implement File.read_chunk

I was using `:file.pread` in a project and thought that this might be a nice addition to `File`. This way `:file.open` and `:file.close` are handled inside of the function. 

If this is a wanted feature then I would be happy to also implement `File.write_chunk`.
Since we only use a subset of these location types we should show it in the typespec.