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/where.go

@@ -18,7 +18,6 @@ import (
 	"fmt"
 	"reflect"
 	"strings"
-	"time"
 )
 
 // Where returns a filtered subset of a given data type.
@@ -404,6 +403,16 @@ func toInt(v reflect.Value) (int64, error) {
 	return -1, errors.New("unable to convert value to int")
 }
 
+func toUint(v reflect.Value) (uint64, error) {
+	switch v.Kind() {
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		return v.Uint(), nil
+	case reflect.Interface:
+		return toUint(v.Elem())
+	}
+	return 0, errors.New("unable to convert value to uint")
+}
+
 // toString returns the string value if possible, "" if not.
 func toString(v reflect.Value) (string, error) {
 	switch v.Kind() {
@@ -415,12 +424,6 @@ func toString(v reflect.Value) (string, error) {
 	return "", errors.New("unable to convert value to string")
 }
 
-var (
-	zero      reflect.Value
-	errorType = reflect.TypeOf((*error)(nil)).Elem()
-	timeType  = reflect.TypeOf((*time.Time)(nil)).Elem()
-)
-
 func toTimeUnix(v reflect.Value) int64 {
 	if v.Kind() == reflect.Interface {
 		return toTimeUnix(v.Elem())