title: Library construct, struct("string"/anythingElse) issue
body: <p>Construct is a parsing library to parse hex datas. After reading the data, you first have to define the map and then parse it. You can find an example below;</p>
  
  <pre><code>....
  data, addr = self.s.recvfrom(1024)
  
  c = Struct("sync"/Int16ul, "ID"/Float32l)
  
  x = c.parse(data)
  
  print(x.sync)
  print(x.ID)
  </code></pre>
  
  <p>My problem here is I'am reading the data types, for our example Int16ul and Float32l from a list named "varBytes" programatically and their type is "str" as expected. Data names "sync" and "ID" is also in a list named "varNames".
  Below is what I have tried;</p>
  
  <pre><code>c = Struct(varNames[0]/varBytes[0],varNames[1]/varBytes[1])
  </code></pre>
  
  <p>This is not working. How can I solve this problem?</p>