Improve debugging of pointer values

[brycekahle]

See #1989

This displays the address of the variable the pointer is pointing to (for pointer comparison). It also allows expansion in the debugger.

Example code:

package main

import "testing"

func TestX(t *testing.T) {
	var i, j int
	i = 0
	j = 1
	k := []string{"asdf"}
	var p = &i
	var q = &j
	var r = &k
	t.Logf("%#v %#v %#v", p, q, r)
}

Shown in debugger:

[brycekahle]

This does add an intermediate level to the expansion. Alternatively, I could attempt to rework the code to display it like so:

p: 0 <*int>(0xc000078080)
q: 1 <*int>(0xc000078088)
r: <*[]string>(0xc000086080)
  [0]: "asdf"

[ramya-rao-a]

Re-working this to look like
#2075 (comment) would be simple for simple types like int, but would get complicated for arrays and maps I believe.

The current solution should be good enough.

Thanks!