Invoke String when formatting map keys

[crawshaw]

This reverts a change introduced in commit 2940eda where the
cmp package stopped calling the String method when printing map
keys. The motivation for the change is unclear, indeed there was
a pre-existing test that String was called on map keys that the
commit changed.

Fixes #141

[dsnet]

I remember now why I avoided String for map keys. It was because of the following situation:

type Int int
func NewInt(i int) *Int { return (*Int)(&i) }
func (i *Int) String() string { return strconv.Itoa(int(*i)) }

m := map[*MyInt]string{
    NewInt(5): "foo",
    NewInt(5): "bar",
}

The stringer for both keys prints to the same thing, which is slightly different from what the map is comparing on (the pointers).

However, that situation should be fixed by a more general change in the future that avoids stringer altogether if the outputs are ambiguous:

go-cmp/cmp/report_compare.go

Lines 18 to 20 in 6f77996

	// TODO: Enforce unique outputs?
	// * Avoid Stringer methods if it results in same output?
	// * Print pointer address if outputs still equal?