message: Make gtl/int_type.h types hashable in ABSL containers.
  (Note ABSL hash functions are templated, and so there's no cost if you don't use them.)
diff: @@ -197,6 +197,11 @@ class IntType {
  }
  };
  + template <typename H>
  + friend H AbslHashValue(H h, const IntType& i) {
  + return H::combine(std::move(h), i.value());
  + }
  +
  public:
  // Default c'tor initializing value_ to 0.
  constexpr IntType() : value_(0) {}
  