Support TransformObject as a reducer type

[raingerber]

TransformMap is currently used for hash entities, but it should be supported as a reducer type (meaning it could be part of a TransformExpression). This would make it easier to manipulate objects without creating full-blown hash entities. One main use case would be combining multiple requests into a single object:

{
  'entry:example': [
    {
      a: 'request:a',
      b: 'request:b'
    },
    // this transform could access the data from requests 'a' and 'b'
    'transform:data'
  ]
}