Rename HasComponents trait to HasProvider

[soareschen]

This PR renames the HasComponents trait to HasProvider. This is to clarify the relationship between a consumer trait and a provider trait, and what it implies when the trait is implemented by a context.

The name HasProvider better reflects what is happening, which is that the CGP context delegates the implementation of all its consumer traits to the specified provider.

Before:

pub trait HasComponents {
    type Components;
}

pub trait HasProvider {
    type Provider;
}