function writeFixed32(val, buf, pos) {
    buf[pos    ] =  val         & 255;
    buf[pos + 1] =  val >>> 8   & 255;
    buf[pos + 2] =  val >>> 16  & 255;
    buf[pos + 3] =  val >>> 24;
}
Writes a boolish value as a varint.
@param {boolean} value Value to write
@returns {Writer} `this`