Based on the Rust clippy lint that recommends that any public
function that dereferences a raw pointer, mark all FFI functions
that reference raw pointers with build_slice and cast_pointer
as unsafe.

This commits starts by removing the unsafe wrapper inside
the build_slice and cast_pointer macros then marks all
functions that use these macros as unsafe.

Instead use Box::into_raw and from_raw which is more idiomatic
for this purpose.  For now do this for Rust objects that are
opaque to C, but have their life time (allocation and free)
managed from C.

As of Rust 1.30 the primary location of c_void is inside
std::ffi and not std::os::raw which is for platform specific
types.