There are two changes in this commit.

The first change builds the list from a variadic append operation
from right to left, which is the natural order for list construction.
This speeds up the following program:

```
(time
 (void
  (append* (for/list ([i 30000])
             (if b '() '(1))))))
```

from 5 seconds to 1 second.

The second change avoids making an assertion when doing unsafe/append
on two symbolic unions. This is fine because all calls to unsafe/append
is already guarded via type casting to make them symbolic unions
of rosette-contract?, so there is no need to add another guard.

Thanks to @camoy for raising up the issue about the unnecessary assertions.