Scale life-on-hit effects to weapon attack speed

Right now the "life on hit" effect seems to favor faster weapons disproportionately. This can be taken into account when the source of lifesteal comes from the weapon itself, but it punishes slower weapons greatly when the lifesteal comes from skills or armors or accessories.

I propose that this stat scales based on how fast the weapon attacks instead. So slower weapons get a bigger chunk of lifesteal per-hit. The overall lifesteal amount across time should be roughly the same for fast and slow weapons.