Remove buf::Source in favor of buf::IntoBuf

The `Source` trait was essentially covering the same case as `IntoBuf`,
so remove it.

While technically a breaking change, this should not have any impact due
to:

1) There are no reverse dependencies that currently depend on `bytes`
2) Source was not supposed to be implemented externally
3) IntoBuf provides the same implementations as `Source`

Given these points, the change should be safe to apply.

src/buf/buf.rs

@@ -720,3 +720,31 @@ impl<T: AsRef<[u8]>> Buf for io::Cursor<T> {
         self.set_position(pos as u64);
     }
 }
+
+impl Buf for Option<[u8; 1]> {
+    fn remaining(&self) -> usize {
+        if self.is_some() {
+            1
+        } else {
+            0
+        }
+    }
+
+    fn bytes(&self) -> &[u8] {
+        self.as_ref().map(AsRef::as_ref)
+            .unwrap_or(Default::default())
+    }
+
+    fn advance(&mut self, cnt: usize) {
+        if cnt == 0 {
+            return;
+        }
+
+        if self.is_none() {
+            panic!("overflow");
+        } else {
+            assert_eq!(1, cnt);
+            *self = None;
+        }
+    }
+}