Upcoming changes to trait API

[SanderMertens]

The trait API is going to be overhauled with new terminology better suited for using it as an entity-relation API. Additionally, a number of inconsistencies between the C and C++ API will be addressed, and a cleaner query syntax will be added. The old trait API will still be supported, but deprecated.

The new API introduces the following terms:

• subject: the entity to which to add a pair (for example: "Bob")
• relation: the kind of relation (for example: "Likes")
• object: the entity which the relation targets (for example: "Alice")
• pair: A combination of both a relation and an object
• term: A single element of a query (currently called 'column')

The C API changes will be as follows (not exhaustive):

// Old
ecs_add_trait(world, subject, object, relation);
// New
ecs_add_pair(world, subject, relation, object);

// Old
ecs_set_trait(world, subject, object, relation, { T = relation });
// New
ecs_set_pair(world, subject, relation, object, { T = relation });

// Old
ecs_set_trait_tag(world, subject, relation, object { T = object });
// New
ecs_set_pair_object(world, subject, relation, object, { T = object });

The iterator API will start using "term" instead of "column":

// Old
Position *p = ecs_column(it, Position, 1);
// New
Position *p = ecs_term(it, Position, 1);

The CHILDOF and INSTANCEOF roles will be translated to relations. Note that ECS_INSTANCEOF will be renamed to EcsIsA:

// Old
ecs_add_entity(world, e, ECS_CHILDOF | parent);
// New
ecs_add_pair(world, e, EcsChildOf, parent);

// Old
ecs_add_entity(world, e, ECS_INSTANCEOF | base);
// New
ecs_add_pair(world, e, EcsIsA, base);

The C++ API changes will be as follows (not exhaustive):

// Old
subject.add_trait<relation, object>();
// New
subject.add<relation, object>();

// Old
subject.add_trait(relation, object);
// New
subject.add(relation, object);

// Old
subject.set_trait<relation, object>({ T = relation });
// New
subject.set<relation, object>({ T = relation });

// Old
subject.set_trait_tag<relation, object>({ T = object });
// New
subject.set_object<relation, object>({ T = object });

The iterator will start using term instead of column:

// Old
auto p = it.column<Position>(1);
// New
auto p = it.term<Position>(1);

The add_instanceof and add_childof methods will be deprecated:

// Old
e.add_instanceof(base);
// New
e.add(flecs::IsA, base);

// Old
e.add_childof(parent);
// New
e.add(flecs::ChildOf, parent);

The revised query language will have these forms for querying:

// Regular component query (same as current)
component

// Get component from an explicit source (an entity, or modifier such as parent, shared, cascade)
component(source)

// query for a pair
(relation, object)

// query for a pair from an explicit source
relation(source, object)

The changes to the query language will be as follows (not exhaustive):

// Old
TRAIT | relation
TRAIT | relation > *
// New
(relation, *)

// Old
TRAIT | relation > object
relation FOR object
// New
(relation, object)

// Old
PARENT:component
// New
component(Parent)

// Old
MyEntity:component
// New
component(MyEntity)

// Old
MyEntity:TRAIT | relation > object
// New
relation(MyEntity, object)

[SanderMertens]

The PR is up: #358