This commit adds support for replacing the compressor used for
serialized cache entries.  Custom compressors must respond to `deflate`
and `inflate`.  For example:

  ```ruby
  module MyCompressor
    def self.deflate(string)
      # compression logic...
    end

    def self.inflate(compressed)
      # decompression logic...
    end
  end

  config.cache_store = :redis_cache_store, { compressor: MyCompressor }
  ```

As part of this work, cache stores now also support a `:serializer`
option.  Similar to the `:coder` option, serializers must respond to
`dump` and `load`. However, serializers are only responsible for
serializing a cached value, whereas coders are responsible for
serializing the entire `ActiveSupport::Cache::Entry` instance.
Additionally, the output from serializers can be automatically
compressed, whereas coders are responsible for their own compression.

Specifying a serializer instead of a coder also enables performance
optimizations, including the bare string optimization introduced by cache
format version 7.1.