Remove unsound casting functions

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)
}
```

src/README.md

@@ -11,7 +11,7 @@ Root Sources
 | `tokenizer.rs` | Script tokenizer/lexer                                                          |
 | `parser.rs`    | Script parser                                                                   |
 | `optimizer.rs` | Script optimizer                                                                |
-| `unsafe.rs`    | `unsafe` functions                                                              |
+| `reify.rs`     | Utilities for making generic types concrete                                     |
 | `tests.rs`     | Unit tests (not integration tests, which are in the `rhai/tests` sub-directory) |