setUint16 = (encoder, pos, num) => {
  set(encoder, pos, num & binary.BITS8);
  set(encoder, pos + 1, num >>> 8 & binary.BITS8);
}
Write two bytes as an unsigned integer at a specific location.

@function
@param {Encoder} encoder
@param {number} pos The location where the data will be written.
@param {number} num The number that is to be encoded.