Code typed into an embedded IPython shell is compiled as module-level
code, so lambdas, generator expressions, comprehensions and function
bodies defined interactively look their free variables up in globals()
and could not see the local variables of the frame the shell was
embedded in:

    def f():
        x = 1
        IPython.embed()   # (lambda: x)() -> NameError

Fix this by giving the embedded shell a globals namespace (a dict
subclass installed via make_main_module_type) that resolves reads
through the caller's local namespace first, then its own snapshot of
the module globals, then the live module dict, mimicking the closure
lookup the code would have had if written in place. Passing a dict
subclass to exec disables the exact-dict LOAD_GLOBAL fast path, which
is what makes the __getitem__ override effective.

STORE_GLOBAL bypasses __setitem__ and mutates the C-level dict storage
directly, so 'global' assignments made by shell-defined functions land
in the snapshot; they are propagated back to the real module when the
shell exits.

Closes gh-136