Nettle: Added __builtin.Nettle.BlockCipher.
Removed optimization of Yarrow seed files for Nettle < 2.0
More Yarrow simplifications.
Documentation update.
Runtime: Added PROGRAM_CLEAR_STORAGE.

Adds a flag indicating that the storage held by objects of the class
should be cleared prior to being freed. This is mainly to avoid having
secrets around longer than necessary.

FIXME: Consider adding a corresponding flag for objects to mark eg
some bignums for clearing.