Add Map support in Avro compiled semantics plugin

[deaktator]

Allow map variables to be extracted as final field in a variable path. Currently, nothing should be allowed to come after a Map object.

Based on the Avro map spec , only maps with string based keys are allowed. Most likely, these Strings are actually Utf8 instances and the Maps are probably java.util.Map instances. These should be turned into Scala Maps with some function like:

def toMap[V](m: java.util.Map[Utf8, V]): Map[String, V] = ???

Example Avro Protocol (example.avpr)

{
  "namespace": "a.b",
  "protocol": "Example",
  "doc": "Example protocol",
  "types": [
    { "name": "X", "type": "record",
      "fields": [ { "name": "m", "type": {"type": "map", "values": "long"} } ]
    },
    { "name": "Msg", "type": "record",
      "fields": [ 
        { "name": "x", "type": ["null", "X"] },
        { "name": "m", "type": {"type": "map", "values": "Msg"} }
      ]
    }
  ]
}

Examples of Acceptable Aloha Feature Specifications

{ "name": "sum_key_lengths", "spec": "(0 /: ${x.m}.keys)(_ + _.length)" }

{ "name": "sum_values", "spec": "${x.m}.values.sum" }

Examples of Unacceptable Aloha Feature Specifications

{ "name": "sum_values", "spec": "${m.x}" }

{ "name": "sum_values", "spec": "${m[msg1]}" }

{ "name": "sum_values", "spec": "${m[msg1].x}" }