Add rbtree_walk() utility and dynablock size histogram at exit

[runlevel5]

While working on the PPC64LE dynarec, I added a small debugging aid that I think would be useful upstream as well.

What this adds:

1. rbtree_walk() — a generic in-order tree traversal function for rbtree_t that calls a user-provided callback for each node with (start, end, data, userdata). This is a read-only utility alongside the existing rbtree_print().

2. Dynablock size histogram at exit — when LOG_INFO is enabled and DYNAREC is compiled in, FreeBox64Context() prints a histogram of live dynablock sizes (by x86 code size) before tearing down db_sizes. It shows the distribution across 12 buckets (1-8 bytes through 8193+), plus total block count, total bytes, max size, and average size.

Example output:

BOX64 Dynarec block size histogram (14523 live blocks, 1843200 total x64 bytes, max=6144, avg=126):
       1-8 bytes:     3241 blocks (22.3%)
      9-16 bytes:     2856 blocks (19.7%)
     17-32 bytes:     2190 blocks (15.1%)
     33-64 bytes:     1987 blocks (13.7%)
    65-128 bytes:     1654 blocks (11.4%)
   129-256 bytes:     1203 blocks ( 8.3%)
   257-512 bytes:      782 blocks ( 5.4%)
  513-1024 bytes:      410 blocks ( 2.8%)
 1025-2048 bytes:      142 blocks ( 1.0%)
 2049-4096 bytes:       48 blocks ( 0.3%)
 4097-8192 bytes:       10 blocks ( 0.1%)

This replaces the commented-out dynarec_log line that was already there in FreeBox64Context().

Risk: Very low — it's purely informational logging behind LOG_INFO, only runs at exit, and the rbtree_walk() function is a straightforward read-only traversal of the existing leftmost→successor chain.

Files changed:

• src/tools/rbtree.c — add rbtree_walk()
• src/include/rbtree.h — declare rbtree_walk() with doc comment
• src/box64context.c — add histogram code in FreeBox64Context(), add #include <inttypes.h>

I'm happy to open a PR for this if there's interest.

[ptitSeb]

It would be interesting, but as a disabled code (at least the print of the histogram), like the current disabled code for the memory tracking in custommem.c. I don't want every box64 session to be ended with the full histogram for everyone.