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_test.go

@@ -26,6 +26,25 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+type tstEqerType1 string
+type tstEqerType2 string
+
+func (t tstEqerType2) Eq(other interface{}) bool {
+	return cast.ToString(t) == cast.ToString(other)
+}
+
+func (t tstEqerType2) String() string {
+	return string(t)
+}
+
+func (t tstEqerType1) Eq(other interface{}) bool {
+	return cast.ToString(t) == cast.ToString(other)
+}
+
+func (t tstEqerType1) String() string {
+	return string(t)
+}
+
 type tstCompareType int
 
 const (
@@ -148,6 +167,10 @@ func doTestCompare(t *testing.T, tp tstCompareType, funcUnderTest func(a, b inte
 		{"a", "a", 0},
 		{"a", "b", -1},
 		{"b", "a", 1},
+		{tstEqerType1("a"), tstEqerType1("a"), 0},
+		{tstEqerType1("a"), tstEqerType2("a"), 0},
+		{tstEqerType2("a"), tstEqerType1("a"), 0},
+		{tstEqerType2("a"), tstEqerType1("b"), -1},
 	} {
 		result := funcUnderTest(test.left, test.right)
 		success := false