What's the best way to represent System.Decimal in Protocol Buffers?

message Decimal {
  optional uint64 lo = 1; // the first 64 bits of the underlying value
  optional uint32 hi = 2; // the last 32 bis of the underlying value
  optional sint32 signScale = 3; // the number of decimal digits, and the sign
}

message Decimal {

    // 96-bit mantissa broken into two chunks
    optional uint64 mantissa_msb = 1;
    optional uint32 mantissa_lsb = 2;

    required sint32 exponent_and_sign = 3;
}