dropping Mwrap::HeapPageBody memalign tracking

[Eric Wong <e@80x24.org>]

Ruby 3.1 switched to mmap for HPB in 2021, and I doubt it'll be
going back to *memalign on Linux or FreeBSD.  HPB stats don't
exist at all for 3.1+ right now.

Tracking mmap allocations safely would be significantly more
difficult and expensive since munmap can cross mmap-ed regions.

With *memalign + free, there's a simple 1:1 relationship,
but not with mmap + munmap.

munmap can work on any subset (or even superset if multiple mmap
calls return sequential pages) of addresses within any mmap-ed
region(s).  In other words, each 4k page would need a
separately-allocated tracking struct in a process-wide tree or
hash table.

I don't think Ruby currently does asymmetric mmap/munmap; but
extensions and any spawned processes may and it's the only safe
way to account for it.

So the tracking is definitely doable, but I'm not sure it's
worth the time and effort.  These are GC-internal allocations
and any instrumentation for the GC itself is probably better off
being added to ruby/gc.c



There's something similar on the Perl 5 side, too.  It allocates
small strings out of 4080-byte malloc-ed arenas and I was
confused with 4080-byte allocations until I cranked up C
backtraces via MWRAP=bt:$N.

I think a better long-term feature would be to be able to
interactively crank up C backtrace levels on a per-callsite
basis.  Right now, the C backtrace level is global, and
increasing that interactively gets expensive fast.