Fix panic calling $distinct on array of arrays (#34)

This fixes the following panic:
```
panic: runtime error: hash of unhashable type []interface {}
goroutine 176 [running]:
http://github.com/xiatechs/jsonata-go/jlib.Distinct ({0x1087cc0?, 0xc000661368?, 0x0?})
	/go/pkg/mod/github.com/xiatechs/[email protected]/jlib/array.go:61 +0x3e5
reflect.Value.call({0x10c10a0?, 0x1404a18?, 0x4cadbf?}, {0x1318275, 0x4}, {0xc000661590, 0x1, 0x0?})
	/usr/local/go/src/reflect/value.go:596 +0xce5
reflect.Value.Call({0x10c10a0?, 0x1404a18?, 0x1?}, {0xc000661590?, 0xc000661590?, 0x1087d00?})
	/usr/local/go/src/reflect/value.go:380 +0xb9
```

jlib/array.go

@@ -45,14 +45,14 @@ func Distinct(v reflect.Value) interface{} {
 		for i := 0; i < items.Len(); i++ {
 			item := jtypes.Resolve(items.Index(i))
 
-			if jtypes.IsMap(item) {
-				// We can't hash a map, so convert it to a
+			if jtypes.IsMap(item) || jtypes.IsArray(item) {
+				// We can't hash a map or array, so convert it to a
 				// string that is hashable
-				mapItem := fmt.Sprint(item.Interface())
-				if _, ok := visited[mapItem]; ok {
+				unhashableItem := fmt.Sprint(item.Interface())
+				if _, ok := visited[unhashableItem]; ok {
 					continue
 				}
-				visited[mapItem] = struct{}{}
+				visited[unhashableItem] = struct{}{}
 				distinctValues = reflect.Append(distinctValues, item)
 
 				continue

jsonata_test.go

@@ -8028,6 +8028,28 @@ func TestTransform(t *testing.T) {
 	})
 }
 
+func TestUnhashableDistinct(t *testing.T) {
+	runTestCases(t, testdata.address, []*testCase{
+		{
+			Expression: `$distinct([["a", "b"]])`,
+			Output: []interface{}{
+				[]interface{}{
+					"a",
+					"b",
+				},
+			},
+		},
+		{
+			Expression: `$distinct([{"a": "b"}])`,
+			Output: []interface{}{
+				map[string]interface{}{
+					"a": "b",
+				},
+			},
+		},
+	})
+}
+
 // Helper functions
 
 type compareFunc func(interface{}, interface{}) bool