feat: remove impl IntoBuf for Cursor<Self>, impl Buf for Bytes, BytesMut, refactor iterators

tests/test_take.rs

@@ -5,7 +5,7 @@ use std::io::Cursor;
 
 #[test]
 fn long_take() {
-    // Tests that take with a size greater than the buffer length will not
+    // Tests that get a take with a size greater than the buffer length will not
     // overrun the buffer. Regression test for #138.
     let buf = Cursor::new(b"hello world").take(100);
     assert_eq!(11, buf.remaining());