Aliases should take precedence over builtin commands

In bash and zsh, you can do aliases like this:

```bash
$ alias ls='ls -l'
$ ls # actually invokes `ls -l`
drwxr-xr-x 5 1000 1000 4096 Jun 9 05:25 dir1
drwx------ 3 1000 1000 4096 Jan 15 13:43 dir2
drwxrwxr-x 2 1000 1000 4096 Jan 15 13:42 dir3
drwxrwxr-x 3 1000 1000 4096 Sep 18 18:20 dir4
```

This is not currently possible in cash, since it looks like calling `ls` immediately invokes the builtin `ls` command (no options).

Also, when fixing this, we should make sure to test for the recursive case. If we use `alias ls='ls -l'`, calling `ls` should invoke the alias, which invokes the builtin.

On the other hand, if we have something more complex:

```bash
$ alias ls='ls -l'
$ alias foo='ls'
$ foo # this ultimately invokes `ls -l`
```

We should make sure to evaluate aliases in such a way that they don't cause cycles.