Magic numbers

Allow embedding magic numbers. Should look like:

```rust
// string data
struct MagicString;
// binary data
struct MagicNumber;

// serializes as `FOO!`
// can only be deserialized from buffer containing `FOO!`
struct Foo {
magic: MagicString<"FOO!">,
}
```

The issue names no files, tests, or entry points. Start by locating the Rust serialization and deserialization implementation, then determine how const-generic string and byte-array markers would fit; done means the shown declarations serialize and reject buffers with incorrect magic data.

Written by the indexing model from the issue text.