… modules

When no objects are found to be imported from external modules, `_stash_modules()` should return `main_module` unmodified, and not the pair list of objects created from it (`original`).

…was imported

Just calling `import multiprocessing` creates a `'__mp_main__'` entry in `sys.modules` that is simply a reference to the `__main__` module. In the old version of the test for module membership, objects in the global scope are wrongly attributed to this virtual `__mp_main__` module. And therefore `load_session()` fails. Testing for object identity instead of module name resolves the issue.

…e in `dump_session(byref=TRUE)`

Currently, `dump_session(byref=True)` misses some imported objects. For example:

- If the session had a statement `import numpy as np`, it may find a reference to the `numpy` named
as `np` in some internal module listed in `sys.resources`.  But if the module was imported with a
non-canonical name, like `import numpy as nump`, it won't find it at all. Mapping the objects by id
in `modmap` solves the issue.  Note that just types of objects usually imported under an alias must
be looked up by id, otherwise common objects like singletons may be wrongly attributed to a module,
and such reference in the module could change to a different object depending on its initialization
and state.

- If a object in the global scope is a top level module, like `math`, again `save_session` may find
a reference to it in another module and it works. But if this module isn't referenced anywhere else,
it won't be found because the function only looks for objects inside the `sys.resources` modules and
not for the modules themselves.

This commit introduces two new attributes to session modules saved by reference:
- `__dill_imported_as`: a list with (module name, object name, object alias in session)
- `__dill_imported_top_level`: a list with (module name, module alias in session)

I did it this way for forwards (complete) and backwards (partial) compatibility.

Oh, and I got rid of that nasty `exec()` call in `_restore_modules()`! ;)

Fixes RecursionWarning error where a function defined in the top level of the module being saved with
`dump_session(byref=True)`, of which "globals" is a reference to the original module's `__dict__`,
makes dill to try to save this instead of the modified module's `__dict__`, triggering recursion.

…ome reason

…ng in a new session

…rder