code: function mapping (type, stateDefinitions, contractName) {
    var match = type.match(/mapping\((.*?)=>(.*)\)$/)
    var keyTypeName = match[1].trim()
    var valueTypeName = match[2].trim()
  
    var keyType = parseType(keyTypeName, stateDefinitions, contractName, 'storage')
    var valueType = parseType(valueTypeName, stateDefinitions, contractName, 'storage')
  
    var underlyingTypes = {
      'keyType': keyType,
      'valueType': valueType
    }
    return new MappingType(underlyingTypes, 'location', util.removeLocation(type))
  }
docs: mapping decode the given @arg type
  
  @param {String} type - type given by the AST
  @return {Object} returns decoded info about the current type: { storageBytes, typeName}