This commit adds two new Lua functions:
 - `box.malloc.info()` is a public function that returns the total
   memory usage of internal data structures allocated with `malloc()`.
   This information is extracted from the `malloc_info()` output.
   See the doc bot request for details.
 - `box.malloc.internal.info()` is an internal function that returns
   a parsed XML document returned by `malloc_info()` as is. It isn't
   documented. We may need it to get the detailed information about
   the system memory allocator.

Both functions are useful only on Linux and only if ASAN is disabled
because otherwise `malloc_info()` is unavailable. On other platforms
`box.malloc.info()` reports zero memory usage while `internal.info()`
returns an empty table.

Closes #7311

@TarantoolBot document
Title: Document `box.malloc.info()`

`box.malloc.info()` is a function that returns a table with two fields:

```
tarantool> box.malloc.info()
---
- size: 2498560
  used: 1835726
...
```

The `used` value is the total amount of memory in bytes allocated by
Tarantool for internal data structures with [`malloc()`][1].

The `size` value is the total amount of memory in bytes allocated from
the system by the `malloc()` allocator. It may not be less than `used`.

The function may be used before `box.cfg()` is called.

Currently, the function is available only on Linux. On other systems it
reports zero memory usage (`{size = 0, used = 0}`).

[1]: https://man7.org/linux/man-pages/man3/malloc.3.html