vinyl: add statistic for total size of memory occupied by tuples

Vinyl tuples returned to the user are allocated with malloc. They may be
pinned by Lua indefinitely. Currently, there's no way to figure out how
much memory is occupied by these tuples. This commit adds a statistic to
box.stat.vinyl() that accounts them.

Closes #8485

@TarantoolBot document
Title: Document `memory.tuple` statistic of `box.stat.vinyl()`

The new statistic shows the total size of memory in bytes occupied by
Vinyl tuples. It includes cached tuples and tuples pinned by the Lua
world.