allow field renaming within multiple fields

I was getting a field name collision when cycling over a multiple relationship field, example:

```
{{ streams:cycle stream="clinic" }}
	{{ physicians }}
		{{ id }} -- this would show clinic id, not physician id
	{{ /physicians }}
{{ /streams:cycle }}
```
renaming the clinic id field seemed to break the physician field entirely, probably because the id was required for getting the related items

after copying the rename capability from stream cycle to the multiple field, this is possible:

```
{{ streams:cycle stream="clinic" }}
	{{ physicians rename:id="p_id" }}
		{{ id }} -- clinic id
		{{ p_id }} -- this shows physician id
	{{ /physicians }}
{{ /streams:cycle }}
```