crypto: mark all operator bool as explicit

If operator bool is not marked as explicit and you try to do such:

a == b

where a and b are instances of class with overloaded bool operator,
the behavior is not the waited one: a and b are casted to bool
BEFORE the comparaison! This could lead into non waited situation.

This patch fixes all operator bool() where overloaded.
This also implements PublicKey operators == and != as needed in value.h.
This last showed a typical case explained upper.

Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>