Restore raw support from upstream

[tsachiherman]

Add the support for Raw objects

[algomaxj]

would be good to get a review from @zeldovich too since he did all of our msgp work

[tsachiherman]

would be good to get a review from @zeldovich too since he did all of our msgp work

I tested it and it seems to be working correctly for our use case, but I'll wait to hear from Nickolai as you suggested.

[zeldovich]

If you're planning to use msgp.Raw to implement msgp.Marshaler, you should add Raw.CanMarshalMsg():

https://github.com/algorand/msgp/blob/master/msgp/write.go#L55

and similarly Raw.CanUnmarshalMsg() for msgp.Unmarshaler:

https://github.com/algorand/msgp/blob/master/msgp/read.go#L81

Otherwise, this seems like a reasonable change. I got rid of it as part of a broader removal of unused features from msgp, but this seems like a perfectly fine thing to resuscitate.