Const loader without generics.

[timothee-haudebourg]

This PR tackles the two last issues of the Loader trait:

• its type parameter.
• its mutable nature.

The type parameter is annoying because it basically bounds the loader to a specific Vocabulary implementation, but there are many cases where we want to define a loader without committing to a specific vocabulary. The result is often lots of trait bounds everywhere that could be avoided.

The fact that load takes a &mut self is completely unnecessary. None of the provided loader implementations are mutating anything when load is called. The only case where it could be useful is for a loader implementing a cache, but even then we can argue that a cache should not change the functional nature of load and should be implemented with interior mutability.

The fact that the loader must be passed with a mutable reference is annoying in many places where we could benefit from providing an immutable Copy reference.