refactor: reduce malloc in encoding/decoding

[github-actions[bot]]

Read:

+--------------------------------------------+----------+---------+
|                                            | tonbo    | rocksdb |
+=================================================================+
| random range reads                         | 311825ms | 38276ms |
|--------------------------------------------+----------+---------|
| random range reads                         | 312582ms | 37954ms |
|--------------------------------------------+----------+---------|
| random range projection reads              | 240178ms | 37558ms |
|--------------------------------------------+----------+---------|
| random range projection reads              | 241856ms | 37932ms |
|--------------------------------------------+----------+---------|
| random range projection reads (4 threads)  | 225014ms | 38220ms |
|--------------------------------------------+----------+---------|
| random range projection reads (8 threads)  | 197989ms | 38166ms |
|--------------------------------------------+----------+---------|
| random range projection reads (16 threads) | 191792ms | 36593ms |
|--------------------------------------------+----------+---------|
| random range projection reads (32 threads) | 191030ms | 36504ms |
+--------------------------------------------+----------+---------+

[github-actions[bot]]

Write:

+-------------------------------+--------+---------+
|                               | tonbo  | rocksdb |
+==================================================+
| bulk load                     | 6502ms | 9608ms  |
|-------------------------------+--------+---------|
| individual writes             | 8394ms | 10604ms |
|-------------------------------+--------+---------|
| individual writes (4 threads) | 8380ms | 10723ms |
|-------------------------------+--------+---------|
| individual writes (8 threads) | 7898ms | 10616ms |
|-------------------------------+--------+---------|
| batch writes                  | 8101ms | 7312ms  |
|-------------------------------+--------+---------|
| batch writes (4 threads)      | 7594ms | 7299ms  |
|-------------------------------+--------+---------|
| batch writes (8 threads)      | 7627ms | 7271ms  |
|-------------------------------+--------+---------|
| removals                      | 3140ms | 3902ms  |
+-------------------------------+--------+---------+