Negative indexes, closes #274

[odino]

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

[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:

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.