YJIT: Show Context stats on exit

[k0kubun]

As we started to discuss how we compress Context, I'd like to add stats about its size.

Through these metrics, I'd like to remind us that memory consumed by Context is currently not that dominant, but these stats will also be useful to notice it's getting more important as we add more fields to it.

Current stats

Current YJIT stats with stats build on railsbench 25 itrs:

compiled_iseq_count:           2,200
compiled_block_count:         20,675
compiled_branch_count:        36,009
...
code_region_size:          4,259,840
yjit_alloc_size:           9,159,933
live_context_size:         1,021,516
live_context_count:           26,882

Implementation design

I considered incrementing or decrementing the counter as it gets allocated or deallocated, but it seemed rather complicated. Scanning all at the end seems to make the implementation simple and precise. However, because it comes at the cost of scanning all payloads, these metrics are available only at exit by default.

[maximecb]

Good to have these stats 👍