Cleanup in preparation for mempool reference auditing feature in checked builds

Cleanup in preparation for mempool reference auditing feature in checked builds

- Rework in mempool.c: Have USE_MALLOC_FOR_MEMPOOLS work by tweaking existing rather than using an entire different path. Remove unused mempool field “rest”, stop assigning fields “pos” and “end” where unused. Removed unused function mono_mempool_empty. Fix off-by-one bug (>= vs >) in mono_mempool_contains_addr.
- Fix misc bug spotted by Rodrigo in aot-runtime.c: image of MonoGenericParam was always being set to corlib instead of proper image
- Add comments and replace inline constants with named constants in various places