This adds an is_canonic parameter to all sign functions. This is a
callback that determines if a signature corresponds to some coin
specific rules. It is used, e. g., by ethereum (where the recovery
byte must be 0 or 1, and not 2 or 3) and or steem signatures (which
require both r and s to be between 2^248 and 2^255).
This also separates the initialization and the step function of the
random number generator, making it easy to restart the signature
process with the next random number.
