The child stack wouldn't be popped nor freed.

Additionally, even once the packer was freed, only the latest stack
would be freed, all the parent stack would be leaked.

Practically speaking, every time a recursive unpacker would raise,
4kiB would be leaked.

Prevent memory leak when a recursive unpacker raises an exception

Extracted from: #370

As initially implemented, we allocate a new stack whenever we
encounter a child stack. This means acquiring a whole 4kiB chunk
from the arena, which generally is overkill.

Instead we can introduce a new type of stack item, that behave just
like a Hash or Array.

Now that there's no longer a concept of child stacks, we can
embed the struct and save a malloc/free pair.

Reduce its size from 264 to 256B.

Reduce its size from 224 to 216B.

It's only used by the unpacking loop, and can perfectly be allocated
on the stack, no reason to add an extra 8B in every buffer struct for
it.

Remove `cast_block` from `msgpack_buffer`

Reorder msgpack_packer struct

Refactor unpacking of recursive types