Panic when calling receiver methods for non-struct types

[sanjit-bhat]

Goose doesn't yet support receiver methods for non-struct types. However, it's probably bad UX to panic (without saying the unsupported error msg), like it does in this example.

type S []byte

func (s S) Receiver() {
	return
}

func bad() {
	var s S
	s.Receiver()
}

After running Goose, I get the following error. Looks like it's assuming the type is a struct and calling the Go NumFields method on it, which panics.

panic: runtime error: invalid memory address or nil pointer dereference
goroutine 15 [running]:
github.com/tchajed/goose.Ctx.declsOrError.func1()
        /Users/sanjit/projects/pdos/goose/interface.go:27 +0x80
panic({0x10246fee0?, 0x10260aee0?})
        /usr/local/go/src/runtime/panic.go:914 +0x218
go/types.(*Struct).NumFields(...)
        /usr/local/go/src/go/types/struct.go:43
github.com/tchajed/goose.structTypeInfo.fields(...)
        /Users/sanjit/projects/pdos/goose/types.go:367
github.com/tchajed/goose.Ctx.selectorMethod({{0x1400017aba0}, 0x14000025e50, 0x1400007e380, {0x14000018db0, 0x26}, {0x1400007e380}, {0x0, 0x0, {0x1023dfa87, 0x4}}, ...}, ...)
        /Users/sanjit/projects/pdos/goose/goose.go:505 +0xeec
...