generalize eval_passer() for candidates
reuse code of eval_passer() for candidates, which is both
- elegant and simplifies code. the old code for candidates was really ugly and didn't account for relative king position in the endgame, as well as the "quality" of pawn support for the candidate. all these features are now inherited from the more advanced eval_passer()
- future improvements of eval_passer() will naturally impact candidates as well, and this also ensures better eval continuitity (transition candidate to passer).
- a nice ELO improvement: 51.3% after 10000 games in 10"+0.1"

signature = 6665753