rust: remove all usage of transmute

rust: remove all usage of transmute

All cases of our transmute can be replaced with more idiomatic
solutions and do no require the power of transmute.

When returning an object to C for life-time management, use
Box::into_raw to convert the boxed object to pointer and use
Box::from_raw to convert back.

For cases where we're just returning a pointer to Rust managed
data, use a cast.