function toVLQSigned(aValue) {
    return aValue < 0
      ? ((-aValue) << 1) + 1
      : (aValue << 1) + 0;
  }
Converts from a two-complement value to a value where the sign bit is
is placed in the least significant bit.  For example, as decimals:
  1 becomes 2 (10 binary), -1 becomes 3 (11 binary)
  2 becomes 4 (100 binary), -2 becomes 5 (101 binary)