New fluent APIs for C++ queries and systems, removed internal C ecs_new_* functions

[SanderMertens]

New builder APIs

New builder classes have been added to the C++ API. These functions allow for the construction of queries and systems without having to use query strings, while retaining the same level of expressiveness. An example:

auto sys = flecs.system<Pos, Vel>()
  .term<Game>.singleton()
  .term<NPC>()
  .each([](flecs::entity e, Pos& p, Vel& v) { });

More complex queries are possible as well, such as selecting components from parent entities:

auto sys = flecs.system<Pos>()
  .term<Pos>().superset(flecs::ChildOf)
  .iter([](flecs::iter& it, Pos& p) { 
    auto parent_p = it.column<Pos>(2);
    // as usual
  });

It is possible to augment existing template parameters, which allows for using each() in more scenarios:

auto sys = flecs.system<Pos, Pos>()
  .arg(2).superset(flecs::ChildOf) // obtain 2nd Pos component from parent
  .each([](flecs::entity e, Pos& p, Pos& p_parent) { });  

The same API is available for queries, using the query_builder function:

auto q = world.query_builder<Pos, Vel>()
  .term<NPC>()
  .build(); // Explicit build operation enables partial builder construction

A more thorough description of the possibilities will be added to the docs.

Introduction of ecs_system_init

Systems have just joined the rest of the API and can now be constructed with the ecs_system_init operation:

ecs_entity_t pos_id = ecs_component_init(world, &(ecs_component_desc_t){
  .entity.name = "Position",
  .size = sizeof(Position),
  .alignment = ECS_ALIGN(Position)
});

ecs_system_init(world, &(ecs_system_desc_t){
  .entity = { .name = "Move", .add = {EcsOnUpdate} },
  .query.filter.terms = {{pos_id}, {vel_id}}
  .callback = Move
});

Removal of ecs_new_* functions

The ecs_new_* functions such as ecs_new_entity, ecs_new_system, ecs_new_component etc have been removed. While these functions were always intended to be internal, some projects have used them in absence of easy to use non-macro alternatives.

Code that uses these functions should be migrated to use ecs_*_init functions, such as ecs_entity_init, ecs_system_init, ecs_component_init etc. See the documentation in the header for more details.