Improve convert-constants to handle dictionaries

The two main drawbacks for convert-constants are the fact that it
can't handle sets/frozensets (mainly due to the fact that I don't know
how useful this would be to the Haskell code) and that it cannot
export dictionaries.

To fix the second case, the current patch changes the code to support
flattening (potentially nested) dictionaries into single name
space. Yes, this could generate conflicts, but they will be detected
at compile time.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>