Throw when trying to encode bad hex values to Buffer

[gmaclennan]

Currently if you try to pass an invalid hex value as an ID, e.g. if you pass fieldIds: [ 'shhh' ] then encode and decode will work without error, but you will get back an empty string: fieldIds: [ '' ]. This is because Buffer.from('shhh', 'hex') returns an empty Buffer.

I think it makes sense to throw if any of these id fields is invalid. We should do:

function idBufferFromHex(idString) {
  const buf = Buffer.from(idString, hex)
  if (buf.length !== 32) throw new Error('invalid id, must be 32-bytes encoded as hex string')
  return buf

[gmaclennan]

We might want a looser check here. We should definitely check for an empty string, and a string that encodes to an empty Buffer, but do we want more than that. Maybe buf.length >=8 to ensure that we have some degree of uniqueness?