Safely insert type erased component

[Person-93]

Objective

Bevy has a public API for inserting components by id, but it's unsafe. https://docs.rs/bevy/latest/bevy/prelude/struct.EntityCommands.html#method.insert_by_id

Solution

This PR adds a safe wrapper around it. The user creates a ComponentInserter when the type is known. Later, the user can use it to insert the component.

Testing

I added an example in the docs which will be run as a doctest.

[urben1680]

Could you write more about the motivation why you want/need this?

I would also like more unit tests:

• Does dropping work as intended? For both the cases that the BackingPtr is consumed and if not.
• Is the constructor panic safe?

[Person-93]

Could you write more about the motivation why you want/need this?

Like I said, I want a safe way to insert type-erased components. Two use cases:

1. utility AI: I can store a Vec<(SystemId<Entity, f32>, ComponentInserter)> in a resource. Then I can iterate over a list of entities and for each one I run each of the systems and insert the component that returns the greatest number.
2. console input: I can store a HashMap<String, ComponentInserter> in a resource and look up a component with a string that comes from user input.

[Person-93]

I would also like more unit tests:

• Does dropping work as intended? For both the cases that the BackingPtr is consumed and if not.
• Is the constructor panic safe?

I've added the unit tests and I've fixed a segfault when the component is a ZST.