feature: check exhaustiveness of type switches for sealed interfaces

[dkrieger]

sealed interfaces include a no-op private method so that only package-local types can implement the interface. it is therefore possible to check exhaustiveness statically.

example:

type Payload interface {
    sealed()
}

type FooPayload struct {
    Foo string
}

func (p FooPayload) sealed() {}

type BarPayload struct {
    Bar int
}

func (p BarPayload) sealed() {}

// . . .

switch p := somePayload.(type) {
case FooPayload:
    fmt.Println(p.Foo)
case BarPayload:
    fmt.Printf("%d", p.Bar)
}

see https://github.com/alecthomas/go-check-sumtype , which works but does not use analysis, and thus cannot be used with tools like nogo