fixes https://github.com/nim-lang/Nim/issues/25457
Small chunks allocate memory in fixed-size cells. Each cell is
positioned at exact multiples of the cell size from the chunk's data
start, which makes it much harder to support alignment
```nim
sysAssert c.size == size, "rawAlloc 6"
if c.freeList == nil:
sysAssert(c.acc.int + smallChunkOverhead() + size <= SmallChunkSize,
"rawAlloc 7")
result = cast[pointer](cast[int](addr(c.data)) +% c.acc.int)
inc(c.acc, size)
```
See also https://github.com/nim-lang/Nim/pull/12926
While using big trunk, each allocation gets its own chunk
94008531c1