Unify haskell-ident functions

Merge functionality of `haskell-ident-pos-at-point`, `haskell-ident-at-point`, and `haskell-spanable-pos-at-point` into one function. This function should return a cons cell of form `(BOUNDARIES . VALUE)`, where `VALUE` is a string representing found identifier, and `BOUNDARIES` is a cons cell of form `(START . END)` representing left and right bounds of identifier. This function should take optional argument to include surrounding backticks for normal identifiers and parentheses for operators (if they present).
- [ ] merge functionality in single function
- [ ] update all dependent code
- [ ] deprecate old functions