Diff is extremely slow

I tried to use cmp.Diff in a unit test, but for my data, the function is so slow that the unit tests time out. The data in question are two decoded versions of the "Go Regular" font (as described [in the Go blog](https://go.dev/blog/go-fonts) ). The comparison takes more than 16 seconds on a fast laptop.

Here is code to reproduce the problem:

```
package main

import (
"fmt"
"log"

"github.com/google/go-cmp/cmp"

"seehuhn.de/go/sfnt/glyph"

"seehuhn.de/go/pdf"
"seehuhn.de/go/pdf/font"
"seehuhn.de/go/pdf/font/charcode"
"seehuhn.de/go/pdf/font/gofont"
"seehuhn.de/go/pdf/font/opentype"
)

func main() {
otf, err := gofont.OpenType(gofont.GoRegular)
if err != nil {
log.Fatal(err)
}

encoding := make([]glyph.ID, 256)
encoding[65] = otf.CMap.Lookup('A')
encoding[66] = otf.CMap.Lookup('C')

toUnicode := map[charcode.CharCode][]rune{
65: {'A'},
66: {'C'},
}

info1 := &opentype.EmbedInfoSimpleCFF{
Font: otf,
SubsetTag: "UVWXYZ",
Encoding: encoding,
ToUnicode: toUnicode,
}

rw := pdf.NewData(pdf.V1_7)
ref := rw.Alloc()
err = info1.Embed(rw, ref)
if err != nil {
log.Fatal(err)
}

dicts, err := font.ExtractDicts(rw, ref)
if err != nil {
log.Fatal(err)
}
info2, err := opentype.Extract(rw, dicts)
if err != nil {
log.Fatal(err)
}

d := cmp.Diff(info1, info2)
fmt.Println(d)
}
```

The following `go.mod` file shows the exact versions I used:

```
module seehuhn.de/go/issues/001

go 1.21.0

require (
github.com/google/go-cmp v0.5.9
seehuhn.de/go/pdf v0.3.5-0.20230815144317-1dc53998ea9b
seehuhn.de/go/sfnt v0.3.5-0.20230814123027-5af1767be82b
)

require (
github.com/xdg-go/stringprep v1.0.4 // indirect
golang.org/x/exp v0.0.0-20230713183714-613f0c0eb8a1 // indirect
golang.org/x/image v0.7.0 // indirect
golang.org/x/text v0.9.0 // indirect
seehuhn.de/go/dag v0.0.0-20230612165854-b02059e84ec5 // indirect
seehuhn.de/go/dijkstra v0.9.3 // indirect
seehuhn.de/go/postscript v0.3.5-0.20230811095027-86e1857cf919 // indirect
)
```

Since the font data is of moderate size (a few hundred glyphs, each containing a few tens of control points), I would expect the comparison to be much faster than 16 seconds.