I-Frame Overhaul

This is a complete rewrite of the immunity frame mechanic.
Immunity frames now only protect from that one specific source of damage. This affects all entities -- players and mobs. This means a creature can be attacked from multiple players / mobs / blocks and take full damage.

Examples:

• Multiple zombies could gang up on an iron golem to kill it in just a few seconds.
• Multiple players could attack the ender dragon at the same time.
• Fire aspect swords do a little more damage because the fire damage doesn't have to compete with your swings.
• Harming arrows do significantly more damage, because the harming damage is added to the arrow damage.
• Damage from blocks like lava or cactuses stack with effects like poison and wither

A single damage source still can't damage you more than once in 10 ticks (0.5 seconds). So standing in lava or fire won't do any more damage than before, and players cannot attack any faster than before.

This is compatible with RLCraft.