Efficiently support value classes

[searler]

This example

class Wrapper(val underlying: Int) extends AnyVal
case class Test(i:Int,w:Wrapper)
 AvroSchema[Test].toString(true)

Generates a complex record representation

{
  "type" : "record",
  "name" : "Test",
  "fields" : [ {
    "name" : "i",
    "type" : "int"
  }, {
    "name" : "w",
    "type" : {
      "type" : "record",
      "name" : "Wrapper",
      "fields" : [ {
        "name" : "underlying",
        "type" : "int"
      } ]
    }
  } ]
}

It would best if the schema simply represented w by the underlying int type.

Value classes are very useful, providing type-safety and domain operations without any run-time cost..