Active Support's cache have for long been limited because
of its format. It directly serialize its `Entry` object with
`Marshal`, so any internal change might break the format.
The current shortcommings are:
- The minimum entry overhead is quite ridiculous:
`Marshal.dump(ActiveSupport::Cache::Entry.new("")).bytesize # => 107`
- Only the internal `value` is compressed, but unless it's a String, to do so
it first need to be serialized. So we end up with `Marshal.dump(Zlib.deflate(Marshal.dump(value)))`
which is wasteful.
