Partially implements #74.
Securely erases keys from memory after they are no longer needed. Might have a
performance impact, which I haven't measured (perf measurements aren't
generally reliable on laptops).
Thanks to @stoffu for the suggestion to specialize the pod_to_hex/hex_to_pod
functions. Using overloads + SFINAE instead generalizes it so other types can
be marked as scrubbed without adding more boilerplate.
xmr-eric