Search doesn't search records inside arrays

(I noticed this while working on #1069)

```
$ cat input.tzng 
#0:record[r1:record[a:array[string]]]
0:[[[foo;]]]
#1:record[r1:array[record[r2:string]]]
1:[[[foo;]]]

$ zq -t "foo" input.tzng 
#0:record[r1:record[a:array[string]]]
0:[[[foo;]]]
```

Both records should match, but only the first one does. It looks like `(*fieldIter).Next()` needs to step into arrays.


