Implement backward-sexp

[gracjan]

Implement forward-sexp with negative argument. This is going to be useful for haskell-ident-at-point that mostly duplicates this functionality.

[gracjan]

Idea how to do this:

1. Navigate to previous save point that for sure is not in a token, so a whitespace
2. Navigate forward until point is passed.
3. Return the one less sexp.
4. Repeat until done with argument (this could be optimized, optionally).

[gracjan]

Done in #1158.