The casting functions in `unsafe.rs` were unsound (i.e., they allowed
safe code to cause undefined behavior). While they did appear to be used
in a way that wouldn't cause UB the fact that there exists unsound
functions is unsettling.
This commit removes those functions and replaces it with a macro that
performs the same reification - the difference is that the macro call
will also include the checks which are required to prevent UB. A macro
was chosen instead of a function for two reasons:
1. A macro can keep the same code generation whereas a function would
require going through an `Option` which has negative impacts on code
generation (niche values cause poor DCE).
2. There exist other `unsafe` code blocks in the crate and an attempt to
make Rhai 100% safe is completely out-of-scope for this merge
request, so we may as well use `unsafe` in the macro.
Regarding (2) above, I may come back at a later date with a 100% safe
`reify` function but only once the other `unsafe` blocks are removed.
For posterity, said function would look something like:
```rust
fn reify<A: Any, C>(value: A) -> Option<C> {
let mut v = Some(value);
let v: &mut dyn Any = &mut v;
v.downcast_mut::<Option<C>>().map(Option::take)
}
```
