Add support for string slices with ranges containing negative indices

[darrylabbate]

E.g.

• "abcdef"[..-3] should return "abcd"
• "abcdef"[-4..] should return "cdef"

[darrylabbate]

Additionally, this currently segfaults:

$ ~/projects/riff/bin/riff -e '"string"[..-1]'
Segmentation fault: 11