Don't use internal APIs

We currently have two uses of internal API: - [`zeroize::__internal`](https://github.com/ModProg/derive-where/blob/4980b4ed4e7f6c113714c91f932f141b0035ab3a/src/test/zeroize.rs#L55-L56). - [`Eq::assert_receiver_is_total_eq()`](https://github.com/ModProg/derive-where/blob/4980b4ed4e7f6c113714c91f932f141b0035ab3a/src/test/basic.rs#L51-L56). Both are `doc(hidden)`, so they should be considered unstable. Instead we just have to copy the implementation and emit it ourselves.