ptr_meta 0.3.0

A radioactive stabilization of the ptr_meta rfc
Documentation 
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```rust
// Get the associated metadata for pointers
let str = "hello world";
assert_eq!(ptr_meta::metadata(str), str.len());

let slice = &[1, 2, 3, 4, 5] as &[i32];
assert_eq!(ptr_meta::metadata(slice), slice.len());

// Make your own wide pointers from data pointers and metadata
let bytes = [b'h', b'e', b'l', b'l', b'o'];
let ptr = ptr_meta::from_raw_parts::<str>(bytes.as_ptr().cast(), 5);
println!("{} world!", unsafe { &*ptr }); // prints "hello world!"

// Derive Pointee on your own types
#[derive(ptr_meta::Pointee)]

#[repr(transparent)]

struct CoolStr {
    inner: str,
}

impl CoolStr {
    fn print_cool(&self) {
        println!("😎 {} 😎", &self.inner);
    }
}

let ptr = ptr_meta::from_raw_parts::<CoolStr>(bytes.as_ptr().cast(), 5);
let cool = unsafe { &*ptr };
cool.print_cool(); // prints "😎 hello 😎"

// Implement Pointee for trait objects
#[ptr_meta::pointee]

trait Printable {
    fn print(&self);
}

impl Printable for i32 {
    fn print(&self) {
        println!("i32: {self}");
    }
}

let i32_vtable = ptr_meta::metadata(&0i32 as &dyn Printable);
let one_hundred = 100i32;
let printable = ptr_meta::from_raw_parts::<dyn Printable>(
    (&one_hundred as *const i32).cast(),
    i32_vtable,
);
unsafe {
    (*printable).print(); // prints "i32: 100"
}
```