Rename 'constant_time_compare' to 'timing_safe_compare'. refs #2165

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Use bigger random one-time keys in timing_safe_compare. fixes #2165

Follows this advice from Marsh Ray and Solar Designer: https://twitter.com/zooko/status/431105294777597952

Signed-off-by: Daira Hopwood <daira@jacaranda.org>