Refactor collision masks to something like source

[PaulRitter]

to prevent everyone @'ing sloth to make two things hit each other

[metalgearsloth]

For reference https://developer.valvesoftware.com/wiki/Physics_Entities_on_Server_%26_Client#Collision_Groups

tl;dr rather than masks and layers it would be:

• 1 single layer for a fixture
• Separate prototypes define each layer that collides, e.g. Wall <> Thrown_items, Wall <> Bullets