Time API discussion for v3

[randy408]

Suggestions inspired by #265 and previous discussions:

• Add int64_t versions of all float time values in ecs_iter_t, ecs_world_info_t, EcsTimer, EcsRateFilter and only use the integers for calculations

• Change ecs_time_t to a typedef of int64_t or just use int64_t

  • There doesn't appear to be much use of it being an analog of struct timeval/timespec, the seconds member can be int32_t/int64_t depending on platform, high resolution clock functions typically return a 64-bit integer in nanosecond resolution.

• Change ecs_os_get_time() into a function returning ecs_time_t

• Register ecs_time_t in flecs-meta?

There's also the question of what unit to use, micro- or nanoseconds. With microsecond resolution we would be throwing away precision, most high-resolution clocks have a frequency of <1 us (https://stackoverflow.com/a/53472916/12771788). Nanoseconds require dividing/multiplying with even bigger constants, but it could be argued that defines should be used for constants (ECS_MS/SEC?) in either case and the code would look the same.