Implement component inheritance as a storage feature

[SanderMertens]

This PR makes component inheritance work for most flecs APIs, in addition to queries (which already works):

world.component<Base>();
world.component<Foo>().is_a<Base>();
world.component<Bar>().is_a<Base>();

flecs::entity e = world.entity("e")
  .add<Foo>()
  .add<Bar>();

e.has<Base>(); // true
Base& b = e.get<Base>(); // returns first matching

This PR implements the following changes:

• sink component inheritance into the storage, so it becomes available for all APIs
• get, has, owns, each now work with inheritance
• queries for inherited components are now cacheable
• queries for inherited components now use the correct stride when matching a derived type
• inheritance support is added to reflection, so types can inherit from each other