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Negative indexes, closes #274

This PR adds support for negative indexes in arrays / strings.

``` bash
[1,2,3][-1] #3
```

There is one change that might alter existing ABS scripts, and that's
non-existing indexes, for strings, returning an empty string rather than
`null`.

I believe it's an ok thing to break as both will evaluate to `false`
when casted to boolean, and to check whether an index exists one can
simply:

* check the length of the string (`s.len()`)
* check the boolean value of the index (`!!s[idx]`)

both these examples do not break with these changes. I instead admit
that it would be weird to see code such as:

``` bash
if s[idx] == null {
  ...
}
```

rather than

```
if !s[idx] {
  ...
}
```

So I guess this is a change that can go through, as it really shouldn't
impact much of the userbase.

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