compat: make compat a top level module

Now it is accessible using `require('compat')` instead of
`require('tarantool').compat`.

It follows our usual way to expose built-in modules.

It was not done initially due to doubts about projects, which have its
own `myproject/compat.lua` and have modified `package.loaded` to obtain
the project's compat as `require('compat')`.

It is not recommended and should be avoided. Project's modules should be
imported using `require('myproject.<...>')`. Otherwise it may clash with
a future built-in module or any external one.

Follows up #7000

NO_DOC=will be added to https://github.com/tarantool/doc/issues/3259