Variadic form of "append" builtin

There is condition inside current `append` implementation:
```go
if len(args) != 2 {
	panic(exn.NoImpl("variadic append"))
}
```
This means that only `append(S, X)` works.
`emacs/rt` has `SlicePush`, which is a specialization to that particular case.

`append(S, X, YS...)` fails to compile.
`SliceConcat` is required to support that.
