Panic when dumping a struct with a private map field

[bradleyjkemp]

When dumping a struct that has a private field containing a map litter panics because the map key sort function calls .Interface() on an unexported value.
Repro case:

package main

import (
	"github.com/sanity-io/litter"
)

type privateMap struct {
	private map[string]map[string]bool
}

func main() {
	litter.Options{
		HidePrivateFields: false,
		StripPackageNames: true,
	}.Dump(&privateMap{
		private: map[string]map[string]bool{
			"panics": {
				"panics": true,
			},
			"test": {
				"test": true,
			},
		},
	})
}

Results in error:

&privateMap{
  private: map[string]map[string]bool{
panic: reflect.Value.Interface: cannot return value obtained from unexported field or method

With the stack trace attributing this panic to dump.go:392