Rename Buf/BufMut, methods to chunk/chunk_mut (#450)

The `bytes()` / `bytes_mut()` name implies the method returns the full
set of bytes represented by `Buf`/`BufMut`. To rectify this, the methods
are renamed to `chunk()` and `chunk_mut()` to reflect the partial nature
of the returned byte slice.

`bytes_vectored()` is renamed `chunks_vectored()`.

Closes #447

.github/workflows/ci.yml

@@ -11,6 +11,7 @@ on:
 env:
   RUSTFLAGS: -Dwarnings
   RUST_BACKTRACE: 1
+  nightly: nightly-2020-12-17
 
 defaults:
   run:
@@ -120,7 +121,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Install Rust
-        run: rustup update nightly && rustup default nightly
+        run: rustup update $nightly && rustup default $nightly
       - name: Install rust-src
         run: rustup component add rust-src
       - name: ASAN / TSAN