heap chunk_fields struct optimistic

[wu1045718093]

we can use the bottom 2 bits for the current block and prev block used flag. when the previous block flag is 0 means the previous is free, so we can use the previous block store LEFT_SIZE field. after this, we just used extra 4 bytes（big heap mode） to store a using block
refer：https://github.com/mattconte/tlsf

[wu1045718093]

Maximum Managed Memory 16Gb->8Gb（big heap mode）

[wu1045718093]

@andyross

[wu1045718093]

design detail
https://github.com/wu1045718093/zephyr/blob/doc/doc/kernel/memory_management/heap-dump.png