New API functions for creation of flecs API primtives

[SanderMertens]

A new API has been introduced to create entities, components, types, queries, filters and triggers. The new API follows a pattern of an init function (where necessary with an accompanying fini function) that accepts a type that contains the parameters for the creation of that specific structure. For example, an entity can now be created like this:

ecs_entity_t e = ecs_entity_init(world, &(ecs_entity_desc_t){
  .my_name = "my_entity",
  .add = {ecs_id(Position), ecs_id(Velocity), Tag}
});

The new functions that have been introduced so far are:

• ecs_entity_init
• ecs_component_init
• ecs_type_init
• ecs_filter_init
• ecs_query_init
• ecs_trigger_init

The following creation functions have been removed or deprecated:

• ecs_new_entity
• ecs_new_component
• ecs_new_type
• ecs_query_new
• ecs_new_trigger
• ecs_new_prefab
• ecs_new_pipeline

The ecs_new_prefab and ecs_new_pipeline functions can be replaced with ecs_entity_init and ecs_type_init:

// Create prefab by adding the Prefab tag:
ecs_entity_t prefab = ecs_entity_init(world, &(ecs_entity_desc_t){
  .name = "MyPrefab",
  .add = {ecs_id(Position), ecs_id(Velocity), Prefab}
});

// Create pipeline by adding the Pipeline tag:
ecs_entity_t pipeline = ecs_type_init(world, &(ecs_type_desc_t){
  .entity = {
    .name = "MyPipeline",
    .add = {EcsPipeline}
  },
  .ids = {EcsPreUpdate, EcsOnUpdate, EcsPostUpdate} 
});

The same approach will soon be applied to systems with an ecs_system_init function. The C++ API will also be updated in time, with new builder classes that populate the desc types.

A new ecs_term_t datastructure has been introduced to describe filter/query terms. This replaces the existing ecs_sig_column_t type. There is not a 1-1 field mapping from the old to the new structure, as the new structure is designed to accommodate the new query language. As the new fields represent a superset of the old functionality, the old features (such as FromParent) can still be expressed but in a different way. The flecs.h header contains a description of the type, more documentation will follow.

The old ecs_sig_t structure has been removed from the core, and is now merged with ecs_filter_t. This means that queries are now constructed from filters, where a filter is essentially a list of terms. While the filter implementation is not yet updated (it still only reads the include, exclude fields), in time the filter iterator will also use the new term structure, which will significantly increase the expressiveness of filters.

A new trigger API has been introduced which no longer requires importing the system module. Triggers are now a fully integrated part of the core. Triggers are single-term event listeners, where the term describes the interest of the trigger. Currently only basic triggers are supported, but the trigger API will be extended in the future to support the full expressiveness of a term.