tpl/collections: Add Pages support to Intersect and Union

This enables `AND` (`intersect`)  and `OR` (`union`)  filters when combined with `where`.

Example:

```go
{{ $pages := where .Site.RegularPages "Type" "not in" (slice "page" "about") }}
{{ $pages := $pages | union (where .Site.RegularPages "Params.pinned" true) }}
{{ $pages := $pages | intersect (where .Site.RegularPages "Params.images" "!=" nil) }}
```

The above fetches regular pages not of `page` or `about` type unless they are pinned. And finally, we exclude all pages with no `images` set in Page params.

Fixes #3174

tpl/collections/apply.go

@@ -148,3 +148,15 @@ func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
 	}
 	return v, false
 }
+
+func indirectInterface(v reflect.Value) (rv reflect.Value, isNil bool) {
+	for ; v.Kind() == reflect.Interface; v = v.Elem() {
+		if v.IsNil() {
+			return v, true
+		}
+		if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
+			break
+		}
+	}
+	return v, false
+}