Made magicarp fire a random magic projectile each time

*This is @Incoming5643's baby. If he doesn't like it then it probably shouldn't be merged.*

Before this, magicarp would choose a random magic projectile on ```New()``` and would use that, now it chooses a random projectile on ```Shoot()```, which is called every time it shoots.

I also made it randomize from ```initial(projectiletype)``` instead of directly from ```/obj/item/projectile/magic``` so someone can make a guncarp in the future without much hassle.

I left the ```New()``` definition in there for sanity.