Commits on Jan 14, 2019

1. Make set hashing robust to bitwise XOR cancellations …

   The way sets (maps, structs, slices with the "set" tag) are hashed
   relies on an XOR to mix the hashes of their elements. This allows
   multiple independent items to interfere with each other through XOR
   operations. For example, consider a "set" slice with the following two
   structs:
   
   [
     {Key: "a", Value: "v1"},
     {Key: "a", Value: "v2"}
   ]
   
   Changing both keys from "a" to "b" will not change the hash value of the
   overall object, since in either case the two keys will cancel out under
   XOR arithmetic.
   
   This change fixes the problem by adding a step after every set of
   unordered hash operations that "hardens" the result by applying another
   hash. It will prevent components of one unordered hashing operation (for
   example, `Key: "a"` above) from interacting with the same data
   encountered later in a different context.
   
   Note that this changes hashes produced by the package for given inputs.
   
   Closes: mitchellh#18

   Aaron Lehmann committed Jan 14, 2019
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