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/mod.rs

@@ -23,7 +23,6 @@ mod chain;
 mod into_buf;
 mod iter;
 mod reader;
-mod source;
 mod take;
 mod writer;
 
@@ -34,6 +33,5 @@ pub use self::chain::Chain;
 pub use self::into_buf::IntoBuf;
 pub use self::iter::Iter;
 pub use self::reader::Reader;
-pub use self::source::Source;
 pub use self::take::Take;
 pub use self::writer::Writer;