lib/os/heap: straighten up our type usage

The size_t usage, especially in struct z_heap_bucket made the heap
header almost 2x bigger than it needs to be on 64-bit systems.
This prompted me to clean up our type usage to make the code more
efficient and easier to understand. From now on:

- chunkid_t is for absolute chunk position measured in chunk units
- chunksz_t is for chunk sizes measured in chunk units
- size_t is for buffer sizes measured in bytes

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>