Structs should be serializable

[gusevfe]

It would be very useful to be able to serialize fast open structs, since usual open structs allow this. For example, Marshal can be used with regular structs, but not with open ones.

$ irb
irb(main):001:0> require 'ostruct'
=> true
irb(main):002:0> Marshal.dump(OpenStruct.new)
=> "\x04\bU:\x0FOpenStruct{\x00"
irb(main):003:0> require 'fast_open_struct'
=> true
irb(main):004:0> Marshal.dump(FastOpenStruct.new)
TypeError: can't dump anonymous class #<Class:0x20055030>
        from (irb):4:in `dump'
        from (irb):4
        from /usr/bin/irb:12:in `<main>'