Reduce the base by the modulus when the base is larger than
the modulus.  This can unboundedly speed the "startup costs"
of doing modular exponentiation, particularly in cases where
the base is much larger than the modulus.  Original patch
by Armin Rigo, inspired by https://github.com/pyca/ed25519.

frozenset subclasses.

Report and patch by Hrvoje Nikšić

The path wasn't being taken due to an over-restrictive type check.

Patch by Oscar Benjamin.