Make it possible to use go-cmp with autogold.Want inline updates if so desired

[emidoots]

Not unreasonable that someone may have a fair amount of go-cmp tests and want to integrate autogold slowly. We could allow this by having autogold.Want(...).Value() return the interface() value it got, and allowing the user to signal if the test needs updating or not. e.g. they call .Value(), pass it to go-cmp, and invoke .Fail() or something like that.