alloc: reduce page header further, go down to 64k minimum.
[ccan] / ccan / alloc /
drwxr-xr-x   ..
-rw-r--r-- 2805 _info
-rw-r--r-- 29225 alloc.c
-rw-r--r-- 4386 alloc.h
drwxr-xr-x - test