Embedded struct fields are not supported by compiler

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I look at the code generated for embedded fields. So I try testing it and for

--- a/pkg/compiler/struct_test.go
+++ b/pkg/compiler/struct_test.go
@@ -397,6 +397,18 @@ var structTestCases = []testCase{
                `,
                big.NewInt(42),
        },
+       {
+               "embedded struct",
+               `type S11 struct { S10; b int }
+               func F%d() int {
+                       var s S11
+                       s.a = 31
+                       s.b = 11
+                       return s.a + s.b
+               }
+               `,
+               big.NewInt(42),
+       },
        {
                "omit field names",
                `type pair struct { a, b int }

I get

--- FAIL: TestStructs (0.02s)
    --- FAIL: TestStructs/embedded_struct (0.00s)
        vm_test.go:69: 
                Error Trace:    /home/rik/dev/neo-go/pkg/compiler/vm_test.go:69
                                                        /home/rik/dev/neo-go/pkg/compiler/struct_test.go:461
                Error:          Received unexpected error:
                                at instruction 499 (SETITEM): unhandled exception: "The value -1 is out of range."
                Test:           TestStructs/embedded_struct
FAIL
FAIL    github.com/nspcc-dev/neo-go/pkg/compiler        9.481s
FAIL

Describe the solution you'd like

Add proper support for it.