Fix: checker panic on embedded pointer to struct field

[sgreben]

Here's a fix for a case I ran into in practice - the checker panics on a valid input:

expression:

(Embed).EmbedPointerEmbedInt > 0

environment:

type Env struct {
	Embed
}
type Embed struct {
	*EmbedPointerEmbed
}

type EmbedPointerEmbed struct {
	EmbedPointerEmbedInt int
}

crash:

panic: reflect: NumField of non-struct type *mock.EmbedPointerEmbed [recovered]
        panic: reflect: NumField of non-struct type *mock.EmbedPointerEmbed

The fix in this PR is to dereference embedded fields until they stop being pointers (see diff in checker/types.go).

[antonmedv]

Let’s also add a rest what evaluation works and we can call a method on EmbeddedPointerStruct.

[sgreben]

^added (passing) tests for eval & method calls