compiler: emit error for non-byte subslices

They are not supported for now, as VM has only
`SUBSTR` opcode for Buffers (`[]byte` in Go).

pkg/compiler/codegen.go

@@ -517,6 +517,10 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.SliceExpr:
+		if isCompoundSlice(c.typeOf(n.X.(*ast.Ident)).Underlying()) {
+			c.prog.Err = errors.New("subslices are supported only for []byte")
+			return nil
+		}
 		name := n.X.(*ast.Ident).Name
 		c.emitLoadVar("", name)