Use reflect.Value.IsZero (#297)

Now that Go 1.13 is the minimum version, we can use the reflect.Value.IsZero
method instead of our own internal/value.IsZero function.
Interestingly, our IsZero function pre-dates the IsZero method,
but fortunately has the exact same semantics, since both are targetting
semantics defined by the Go language specification.

cmp/report_compare.go

@@ -7,8 +7,6 @@ package cmp
 import (
 	"fmt"
 	"reflect"
-
-	"github.com/google/go-cmp/cmp/internal/value"
 )
 
 // numContextRecords is the number of surrounding equal records to print.
@@ -248,11 +246,11 @@ func (opts formatOptions) formatDiffList(recs []reportRecord, k reflect.Kind, pt
 				var isZero bool
 				switch opts.DiffMode {
 				case diffIdentical:
-					isZero = value.IsZero(r.Value.ValueX) || value.IsZero(r.Value.ValueY)
+					isZero = r.Value.ValueX.IsZero() || r.Value.ValueY.IsZero()
 				case diffRemoved:
-					isZero = value.IsZero(r.Value.ValueX)
+					isZero = r.Value.ValueX.IsZero()
 				case diffInserted:
-					isZero = value.IsZero(r.Value.ValueY)
+					isZero = r.Value.ValueY.IsZero()
 				}
 				if isZero {
 					continue

cmp/report_reflect.go

@@ -184,7 +184,7 @@ func (opts formatOptions) FormatValue(v reflect.Value, parentKind reflect.Kind,
 		}
 		for i := 0; i < v.NumField(); i++ {
 			vv := v.Field(i)
-			if value.IsZero(vv) {
+			if vv.IsZero() {
 				continue // Elide fields with zero values
 			}
 			if len(list) == maxLen {