Make inv_mod2k(_vartime) return a CtChoice

[fjarri]

• inv_mod2k(_vartime) return a tuple with the second parameter indicating whether the inverse exists
• In inv_mod2k(_vartime) docstrings, removed the note about self having to be less than 2^k - the algorithm works just fine even if it's not
• Simplified the usage of inv_mod2k in the creation of DynResidueParams
• inv_odd_mod() now also returns a falsy CtChoice in case of an even modulus