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/lib.rs

@@ -83,7 +83,6 @@ pub use buf::{
 #[deprecated(since = "0.4.1", note = "moved to `buf` module")]
 #[doc(hidden)]
 pub use buf::{
-    Source,
     Reader,
     Writer,
     Take,