Performance Issue Comparing Array Types

I ran into an issue where the comparison of a struct type containing lots of arrays took super long. After adding `cmpopts.EquateComparable` to the check the comparison is more than 10x faster. I think it should be okay to compare arrays of basic types (e.g. `[32]byte`) using `==` by default.

### Example Benchmark:

```go
func BenchmarkCmp(b *testing.B) {
var v0, v1 [32]byte
b.Run("plain", func(b *testing.B) {
for i := 0; i < b.N; i++ {
cmp.Diff(v0, v1)
}
})

b.Run("comparable", func(b *testing.B) {
for i := 0; i < b.N; i++ {
cmp.Diff(v0, v1, cmpopts.EquateComparable([32]byte{}))
}
})

type S struct {
A [32]byte
B [32]byte
C [32]byte
D []struct {
E [32]byte
F [32]byte
}
}
var s0, s1 S
b.Run("struct_plain", func(b *testing.B) {
for i := 0; i < b.N; i++ {
cmp.Diff(s0, s1)
}
})

b.Run("struct_comparable", func(b *testing.B) {
for i := 0; i < b.N; i++ {
cmp.Diff(s0, s1, cmpopts.EquateComparable([32]byte{}))
}
})
}
```
### Results:
```
│ sec/op │
Cmp/plain-8 7.451µ ± 3%
Cmp/comparable-8 4.816µ ± 6%
Cmp/struct_plain-8 32.92µ ± 29%
Cmp/struct_comparable-8 13.56µ ± 5%
```