Adds a declassify operation to aid constant-time analysis.

Adds a declassify operation to aid constant-time analysis.

ECDSA signing has a retry loop for the exceptionally unlikely case
 that S==0.  S is not a secret at this point and this case is so
 rare that it will never be observed but branching on it will trip
 up tools analysing if the code is constant time with respect to
 secrets.

Derandomized ECDSA can also loop on k being zero or overflowing,
 and while k is a secret these cases are too rare (1:2^255) to
 ever observe and are also of no concern.

This adds a function for marking memory as no-longer-secret and
 sets it up for use with the valgrind memcheck constant-time
 test.