Remove IntoBuf/FromBuf (#288)

As consequence Buf::collect is removed as well, which is replaced with `Buf::into_bytes`. The advantage of `Buf::into_bytes` is that it can be optimized in cases where converting a `T: Buf` into a `Bytes` instance is efficient.

src/lib.rs

@@ -75,7 +75,6 @@ pub mod buf;
 pub use crate::buf::{
     Buf,
     BufMut,
-    IntoBuf,
 };
 
 mod bytes;