zeroize: implement `Zeroize` for `PhantomData`/`PhantomPinned`

[tarcieri]

Originally filed by @Allen-Webb as iqlusioninc/crates#651

Since often pinning is desired in conjunction with Zeroize, it would be nice to have zeroize define a Zeroize implementation for core::marker::PhantomPinned so that derive can be used with structs that contain PhantomPinned to prevent moving.

Originally filed by @daxpedda as iqlusioninc/crates#899

Sometimes, when you want to implement Zeroize for a structure that contains a PhantomData field, it's necessary to implement Zeroize by hand. I suggest improving this by simply implementing Zeroize for PhantomData, which should be no-op, as this has no representation data wise I believe.

Alternatively, the derive proc-macro could detect PhantomData and skip it. I prefer the above solution though.

[daxpedda]

@tarcieri, this can be closed, right?

[tarcieri]

Yes