Implement conversion of Python constants to Haskell

With the merge of the repositories, we can now auto-generate the code
for Haskell constants from the Python code.

Currently this only handles the basic types (strings and
integers). Handling containers such as lists and dictionaries is only
possible if we would use a parser such that we recognise the element
names. We could extend the convert-constants script if that becomes
necessary, right now I'm looking at just the simple constants such as
Iallocator modes, instance states, etc.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>