sprintf: removed dead debug check

This debug check has been wrong/dead since the format stack is dynamically
reallocated. Thanks to Chris Angelico <rosuav@gmail.com> for pointing that out.
Runtime: Added convenience function get_inherited_storage().

This function returns the storage in the current object belonging
to a specific inherit.
Nettle.BlockCipher: Added new mode: PCBC.

This adds support for the Propagating Cipher Block Chaning mode, which
is a modified variant of CBC.