expose mi_stats_t via public interfaces

[romange]

It could be nice if we could:

a) (read-only) access thread local stats
b) (read-only) access _mi_stats_main

getting stats via mi_heap_visit_blocks is just too expensive cpu-wise.

[epoupon]

@daanx do you plan to work on this?
(Would also be nice to have some stats per heap)