After adding the autorequiring luatest [1,2], there is no need to use
the following approach now:

```
local t = require('luatest')
local g = t.group()

server:exec(function()
    local t = require('luatest') -- duplicate
    t.assert(...)
end)
```

Modern approach looks like:

```
local t = require('luatest')
local g = t.group()

-- `t` already available in the remote server
server:exec(function() t.assert(...) end)

-- also it works with any variable
local my_custom_t = require('luatest')

server:exec(function()
    my_custom_t.assert(...) -- already available
end)
```

[1] tarantool/luatest#277
[2] tarantool/luatest#289

Part of tarantool/luatest#233

NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix

Changed the work with hooks in the part of tests.

Legacy approach:

```
g.before_all = function() ... end
g.after_all = function() ... end
```

The logic of executing hooks will be expected and
more controlled if use following approach:

```
g.before_all(function() ... end)
g.after_all(function() ... end)
```

Resolve #8066

NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix

Some internal modules have been recently copied to luatest repo [1,2]
and now they can be safely removed, and the corresponding functionality
from luatest can be used instead.

Affected modules:

- test/luatest_helpers/server.lua

[1] tarantool/luatest#258
[2] tarantool/luatest#266

Closes tarantool/luatest#239

NO_DOC=testing stuff
NO_TEST=testing stuff
NO_CHANGELOG=testing stuff

The _vfunc system space is the sysview for the _func system space.
However, the _vfunc format is different from the _func format. This
patch makes the _vfunc format the same as the _func format.

Closes #7822

NO_DOC=bugfix

Closes #3349

@TarantoolBot document
Title: Document that takes_raw_args function option

A new function option was added - `takes_raw_args`:

```lua
box.schema.func.create('my_func', {takes_raw_args = true})
```

If set for a Lua function, the functions arguments will be passed
wrapped in a msgpack object (see `msgpack.object()`) when the function
is called over IPROTO or by `box.func.NAME:call()`:

```lua
local msgpack = require('msgpack')
local my_func = function(mp)
    assert(msgpack.is_object(mp))
    local args = mp:decode() -- array of arguments
end
```

Using this option might improve performance in case a function forwards
most of its arguments to another instance or writes them to a database,
because it eliminates msgpack decoding in Lua.

Currently, `is_multikey` function option must be wrapped in `opts`
sub-table, which looks ugly:

```lua
box.schema.func.create('my_func', {
    is_deterministic = true,
    opts = {is_multikey = true}
})
```

Let's allow to pass the option directly.

Note, the `is_multikey` option is reported by `box.func` at the same
level as `is_deterministic` so this would only be consistent:

```lua
box.func.my_func.is_deterministic
box.func.my_func.is_multikey
```

@TarantoolBot document
Title: Document that is_multikey func opt may be passed without sub-table

It's not necessary anymore to wrap `is_multikey` in `opts` table when
creating a function:

```lua
box.schema.func.create('my_func', {is_multikey = true})
```

is equivalent to

```lua
box.schema.func.create('my_func', {opts = {is_multikey = true}})
```

which still works, but deprecated.

Pease update [this][func-index] documentation entry.

[func-index]: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/create_index/#creating-a-functional-index