This commit renames methods of ServiceProvider to be easier to
understand. It also replaces the misleading singleton with more correct
here global instance of class.
This is intended to be used for intra-module communication. Modules can
specify service that are then implemented by other modules. This way
we can use features from for example part in Core without relying on the
Part module explicitly.
Base does provide Service definition - which is basically an abstract
class with pure virtual methods. This service then can be implemented
in other modules and accessed in runtime via ServiceProvider class that
stores all implementations.
ServiceProvider does store multiple implementations so in theory it is
possible to use it to provide granular implementations. For example,
part can provide CenterOfMass service that provides center of mass for
part features, mesh can implement another one for meshes and then we can
iterate over all implementations and find one that can provide center of
