compare, hugolib, tpl: Add Eqer interface

And use it in `eq` and `ne` so `Page` values can be compared directly in the templates without thinking about it being a `Page` or a `PageOutput` wrapper.

Fixes #3807

tpl/compare/compare.go

@@ -18,6 +18,8 @@ import (
 	"reflect"
 	"strconv"
 	"time"
+
+	"github.com/gohugoio/hugo/compare"
 )
 
 // New returns a new instance of the compare-namespaced template functions.
@@ -85,6 +87,14 @@ func (*Namespace) Default(dflt interface{}, given ...interface{}) (interface{},
 
 // Eq returns the boolean truth of arg1 == arg2.
 func (*Namespace) Eq(x, y interface{}) bool {
+
+	// hugolib.Page implements compare.Eqer to make Page and PageOutput comparable.
+	if e1, ok := x.(compare.Eqer); ok {
+		if e2, ok := y.(compare.Eqer); ok {
+			return e1.Eq(e2)
+		}
+	}
+
 	normalize := func(v interface{}) interface{} {
 		vv := reflect.ValueOf(v)
 		switch vv.Kind() {