OPTIM: ebtree: pack the struct eb_node to avoid holes on 64-bit

OPTIM: ebtree: pack the struct eb_node to avoid holes on 64-bit

struct eb_node is 36 bytes on a 64-bit machine. It's thus rounded
up to 40 bytes, and when forming a struct eb32_node, another 4 bytes
are added, rounded up to 48 bytes. We waste 8 bytes of space on 48
bytes because of alignments. It's basically the same with memory
blocks and immediate strings.

By packing the structure, eb32_node is down to 40 bytes. This saves
16 bytes per struct task and 20 bytes per struct stksess, used to
store each stick-table key.