Add riakc_datatype:context/1 function

[manifest]

Context value of CRDT object could be useful for operations like calculating HTTP Etag. Currently, the only way to retrieve context is to deal with internal representation of CRDT object.

{map,_,_,_,Context} = Obj.

By adding riakc_datatype:context/1, we could make such operations much more reliable.

-callback context(datatype()) -> context().