Rework reading from device and clean up.

src/io.rs

@@ -0,0 +1,15 @@
+use std::io::{self, Seek, SeekFrom};
+
+/// Get length of data stream.
+/// Physical length of data stream in bytes
+/// (multiple of sector_size, rather than actual).
+///
+/// This will attempt to return the stream to its current read position.
+pub fn get_stream_length<S: Seek>(stream: &mut S) -> io::Result<u64> {
+    let pos = stream.stream_position()?;
+    let len = stream.seek(SeekFrom::End(0));
+
+    stream.seek(SeekFrom::Start(pos))?;
+
+    len
+}