compiler: allow to declare slices of compound types

Previously this declarations were ignored which resulted
in runtime errors, because VM's nil is an element of primitive type
and can't be converted to an array.

pkg/compiler/slice_test.go

@@ -3,6 +3,8 @@ package compiler_test
 import (
 	"math/big"
 	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/vm"
 )
 
 var sliceTestCases = []testCase{
@@ -128,6 +130,35 @@ var sliceTestCases = []testCase{
 		}`,
 		[]byte{2, 3},
 	},
+	{
+		"declare compound slice",
+		`package foo
+		func Main() []string {
+			var a []string
+			a = append(a, "a")
+			a = append(a, "b")
+			return a
+		}`,
+		[]vm.StackItem{
+			vm.NewByteArrayItem([]byte("a")),
+			vm.NewByteArrayItem([]byte("b")),
+		},
+	},
+	{
+		"declare compound slice alias",
+		`package foo
+		type strs []string
+		func Main() []string {
+			var a strs
+			a = append(a, "a")
+			a = append(a, "b")
+			return a
+		}`,
+		[]vm.StackItem{
+			vm.NewByteArrayItem([]byte("a")),
+			vm.NewByteArrayItem([]byte("b")),
+		},
+	},
 }
 
 func TestSliceOperations(t *testing.T) {