message: Add missing const on Reference::As<>
  Reference::As<> was needlessly mutable wich made them
  less safe and harder to use
diff: @@ -591,7 +591,7 @@ class Reference {
  }
  }
  - template<typename T> T As();
  + template<typename T> T As() const;
  // Experimental: Mutation functions.
  // These allow scalars in an already created buffer to be updated in-place.
  @@ -704,35 +704,35 @@ class Reference {
  };
  // Template specialization for As().
  -template<> inline bool Reference::As<bool>() { return AsBool(); }
  +template<> inline bool Reference::As<bool>() const { return AsBool(); }
  -template<> inline int8_t Reference::As<int8_t>() { return AsInt8(); }
  -template<> inline int16_t Reference::As<int16_t>() { return AsInt16(); }
  -template<> inline int32_t Reference::As<int32_t>() { return AsInt32(); }
  -template<> inline int64_t Reference::As<int64_t>() { return AsInt64(); }
  +template<> inline int8_t Reference::As<int8_t>() const { return AsInt8(); }
  +template<> inline int16_t Reference::As<int16_t>() const { return AsInt16(); }
  +template<> inline int32_t Reference::As<int32_t>() const { return AsInt32(); }
  +template<> inline int64_t Reference::As<int64_t>() const { return AsInt64(); }
  -template<> inline uint8_t Reference::As<uint8_t>() { return AsUInt8(); }
  -template<> inline uint16_t Reference::As<uint16_t>() { return AsUInt16(); }
  -template<> inline uint32_t Reference::As<uint32_t>() { return AsUInt32(); }
  -template<> inline uint64_t Reference::As<uint64_t>() { return AsUInt64(); }
  +template<> inline uint8_t Reference::As<uint8_t>() const { return AsUInt8(); }
  +template<> inline uint16_t Reference::As<uint16_t>() const { return AsUInt16(); }
  +template<> inline uint32_t Reference::As<uint32_t>() const { return AsUInt32(); }
  +template<> inline uint64_t Reference::As<uint64_t>() const { return AsUInt64(); }
  -template<> inline double Reference::As<double>() { return AsDouble(); }
  -template<> inline float Reference::As<float>() { return AsFloat(); }
  +template<> inline double Reference::As<double>() const { return AsDouble(); }
  +template<> inline float Reference::As<float>() const { return AsFloat(); }
  -template<> inline String Reference::As<String>() { return AsString(); }
  -template<> inline std::string Reference::As<std::string>() {
  +template<> inline String Reference::As<String>() const { return AsString(); }
  +template<> inline std::string Reference::As<std::string>() const {
  return AsString().str();
  }
  -template<> inline Blob Reference::As<Blob>() { return AsBlob(); }
  -template<> inline Vector Reference::As<Vector>() { return AsVector(); }
  -template<> inline TypedVector Reference::As<TypedVector>() {
  +template<> inline Blob Reference::As<Blob>() const { return AsBlob(); }
  +template<> inline Vector Reference::As<Vector>() const { return AsVector(); }
  +template<> inline TypedVector Reference::As<TypedVector>() const {
  return AsTypedVector();
  }
  -template<> inline FixedTypedVector Reference::As<FixedTypedVector>() {
  +template<> inline FixedTypedVector Reference::As<FixedTypedVector>() const {
  return AsFixedTypedVector();
  }
  -template<> inline Map Reference::As<Map>() { return AsMap(); }
  +template<> inline Map Reference::As<Map>() const { return AsMap(); }
  inline uint8_t PackedType(BitWidth bit_width, Type type) {
  return static_cast<uint8_t>(bit_width | (type << 2));
  