Fix failing tests of buffer.

Fix incorrect buffer feature checks

Fortune for chacha20stream's current commit: Future small blessing − 末小吉

src/stream/sink.rs

@@ -39,7 +39,7 @@ use super::*;
 pub struct Sink<W: ?Sized>
 {
     crypter: Crypter, // for chacha, finalize does nothing it seems. we can also call it multiple times.
-    buffer: BufferVec, // used to buffer the operation
+    pub(super) buffer: BufferVec, // used to buffer the operation
 
     stream: W,
 }

src/stream/source.rs

@@ -110,7 +110,7 @@ pub type DefaultBuffer = UseBufferExternal;
 pub struct Source<R: ?Sized, Buffer: ?Sized + BufferKind = DefaultBuffer>
 {
     crypter: Crypter,
-    buffer: Buffer::InternalBuffer, // When `ad-hoc-buffer` is enabled, this isn't needed. We re-use the output buffer for the initial read of untransformed data from `stream` and the actual transformation of the read bytes.
+    pub(super) buffer: Buffer::InternalBuffer, // When `ad-hoc-buffer` is enabled, this isn't needed. We re-use the output buffer for the initial read of untransformed data from `stream` and the actual transformation of the read bytes.
     
     stream: R
 }