Use `u64` for interface handles

[bendk]

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Right now, interface and trait interface handles use raw pointers as their FFI type (*mut ::std::ffi::c_void). This makes sense since they're implemented using Arc::into_raw. This doesn't work great for trait interfaces that have with_foreign support. For those interfaces, we also allow them to be implemented on the foreign side, in which case they are passed across the FFI like a callback interface.

This presents an issue, since callback interfaces normally use u64 for their FFI type. We can make things work by casting the u64 to a pointer, but this isn't ideal:

• It's often awkward to use 2 different FFI types
• It's one of the reasons we have #2567
• There's some possibility of overflow on 32-bit systems.

We should use a single FFI type for this and it seems better to use u64 in all cases than a pointer.