added test and use case of Zeroizing<Point>

(cherry picked from commit e90ee50)

(cherry picked from commit db57eee)

(suggested by @survvived and @elichai)

Note: actual contents of gmp bigint are zeroized on drop within gmp, so it does not make sense to zeroize them by hand.

Note: I had to tweak the implementation of zeroize a bit once again, to get past borrow checker. I believe it still does the same thing, and it still passes my tests, but feel free to double-check.

Note: as pointed out by @survived, passing Scalars by value will create an extra copy on the stack. Several methods and functions were updated to take a reference instead.

Note: as pointed out by @DmytroTym and @survived, moving Scalars into structs will create an extra copy on the stack. Several struct constructions were updated to clone instead.