Rewrite Bytes / BytesMut core implementation

The previous implementation didn't factor in a single `Bytes` handle
being stored in an `Arc`. This new implementation correctly impelments
both `Bytes` and `BytesMut` such that both are `Sync`.

The rewrite also increases the number of bytes that can be stored
inline.

ci/install.sh

@@ -0,0 +1,31 @@
+set -ex
+
+main() {
+    curl https://sh.rustup.rs -sSf | \
+        sh -s -- -y --default-toolchain $TRAVIS_RUST_VERSION
+
+    local target=
+    if [ $TRAVIS_OS_NAME = linux ]; then
+        target=x86_64-unknown-linux-gnu
+        sort=sort
+    else
+        target=x86_64-apple-darwin
+        sort=gsort  # for `sort --sort-version`, from brew's coreutils.
+    fi
+
+    # This fetches latest stable release
+    local tag=$(git ls-remote --tags --refs --exit-code https://github.com/japaric/cross \
+                       | cut -d/ -f3 \
+                       | grep -E '^v[0-9.]+$' \
+                       | $sort --version-sort \
+                       | tail -n1)
+    echo cross version: $tag
+    curl -LSfs https://japaric.github.io/trust/install.sh | \
+        sh -s -- \
+           --force \
+           --git japaric/cross \
+           --tag $tag \
+           --target $target
+}
+
+main