Implement weapon tiers

[Jigoku]

Collecting the same powerup more than once will improve the weapon.
Limit it to 5 tiers.

Random ideas:

2nd tier of cannon could be faster fire rate.
3rd tier could add a rear cannon
4th could add a front/diagonal cannon
5th could add a rear/diagonal cannon

1st tier of rocket will be a single rocket fired
2nd tier, alternating dual rockets
3rd tier, dual homing rockets
4th tier, alternating rockets + homing rockets

Other weapons might just have their fire speed increased. Should make things more interesting, whilst requiring less weapon types to be implemented.

Another idea is that when the player loses a life, remove an entire tier from all weapon types.