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/script.sh

@@ -0,0 +1,18 @@
+# This script takes care of testing your crate
+
+set -ex
+
+main() {
+    cross build --target $TARGET
+
+    if [ ! -z $DISABLE_TESTS ]; then
+        return
+    fi
+
+    cross test --target $TARGET
+}
+
+# we don't run the "test phase" when doing deploys
+if [ -z $TRAVIS_TAG ]; then
+    main
+fi