Fold `get_wet()` into `water_damage()`.

Fold `get_wet()` into `water_damage()`.

This now ensures that dipping into water works like other sources of
water damage. There is a potentially significant gameplay change here:
dipping a container into uncursed water will wet all its contents. If
this is a problem, then we should add another parameter to water_damage
which will suppress this behaviour for dipping.