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