Implement entity archetypes in MVP

[martinjonsson01]

Reason

The MVP should lay out components contiguously in order to demonstrate that it results in efficiency gains due to cache hits.

Requirements

• At first start, entities are grouped into archetypes based on which component types they have attached.
• Systems fetch components from archetypes instead of component vectors.
• Archetypes are constructed in a way such that components that need to be iterated by systems are laid out contiguously (i.e. without empty slots).

Nice-to-have (not strictly necessary):

• The old and naive Vec-based component storage is still there for comparison in benchmarks.

Note: not a requirement: being able to add and remove components and having entities switch archetypes. This is a future feature!

Dependencies

• MVP: Boilerplate #34