Add into_bits convenience method for UperReader (#82)

`UperReader`s can be created from any type implementing `ScopedBitRead`.
However, `UperReader::from` obviously consumes this type.  Users of
`UperReader` may want to get the original `ScopedBitReader` back out
after they are done with the `UperReader`.  This patch provides an
`into_bits` function which returns the underlying `ScopedBitRead` type,
consuming the `UperReader`.

src/syn/io/uper.rs

@@ -751,6 +751,11 @@ impl<'a> From<(&'a [u8], usize)> for UperReader<Bits<'a>> {
 }
 
 impl<B: ScopedBitRead> UperReader<B> {
+    #[inline]
+    pub fn into_bits(self) -> B {
+        self.bits
+    }
+
     #[inline]
     fn read_length_determinant(
         &mut self,