Implement elemental evoker recharge via xp gain

The 4 elemental evokers (fan of gales, lamp of fire, stone of tremors,
and phial of floods) now expend themselves after one use an can be
recharged only by gaining exp while they are in the player's possession.

The formula used almost certainly needs rewriting, as this is only one
step up from a placeholder, but it will hopefully scale with player xl
and evocations skill in a sane enough way to test.

For now, exp gained is split among all evokers, meaning that carrying
two causes each to recharge twice as slowly. I am not certain this is
a good thing, but is up for future adjustment.

1 files changed, 1 insertions, 0 deletions

diff --git a/crawl-ref/source/effects.h b/crawl-ref/source/effects.h
index a70404792d..300f5de080 100644
--- a/crawl-ref/source/effects.h
+++ b/crawl-ref/source/effects.h
@@ -73,6 +73,7 @@ bool vitrify_area(int radius);
 void update_level(int elapsedTime);
 void handle_time();
 void recharge_rods(int aut, bool floor_only);
+void recharge_elemental_evokers(int exp);
 
 void slime_wall_damage(actor* act, int delay);