Renamed identifier to id in function parameters.

Add updated README and docs.

Refactors the internals to provide a cleaner interface.

Binding is simplified; it creates values with either a factory closure with a Container parameter or a value @autoclosure for providing a specific value.

• bind(behavior:type:id:factory: (Container) -> T)
• bind(behavior:type:id:value: @autoclosure () -> T)

Resolving is also simplified with three options.

• resolve(_:id:) -> T? resolves a service, returning nil if it isn't registered
• resolveAssert(_:id:) -> T resolves a service, failing an assertion if it isn't registered
• resolveThrowing(_:id:) throws -> T resolves a service, throwing a FusionError.notRegistered if it isn't registered.

Under the hood changes to singleton storages so that @Inject is equivalent to resolving with a nil identifier.

Replaces already registered instances of singletons when re-registering a service.

Cleans up Inject so that it's get-only, meaning it won't throw errors when used concurrently with Swift's new concurrency features.

Also adds convenience functions for registering an instance of a type without having to specify its actual type.