Test system library modules for persistence

[MikhailArkhipov]

Make representative set, 50-60 modules, make sure save/restore tests pass. Some known have problems, ex: asyncio, datetime, random, pydoc, ctypes, sqlite3, threading.

[MikhailArkhipov]

asyncio issue is part of #1214. When we take function from a stub and it happens to return a generic type, we don't declare the respective TypeVar and end up with type that cannot be resolved. Consider

_F = TypeVar('_F', bound=Callable[..., Any])
def coroutine(func: _F) -> _F: ...

so persistence of asyncio ends up with function returning i:asyncio.coroutines:_F which cannot be resolved from the database. Declaring type is possible, but may conflict with existing types. Consider that module A may be declaring T and importing module B that has a stub that also declares T.

[MikhailArkhipov]

#1429