Affected: crates.io: capnp

The safe API functions `constant::Reader::get` and `StructSchema::new` rely on `PointerReader::get_root_unchecked`, which can cause undefined behavior (UB) by constructing arbitrary words or schemas.

## `Reader::get`

```rust pub fn get(&self) -> Result<<T as Owned>::Reader<'static>> { // ... // UNSAFE: access `words` without validation } ```

## `StructSchema::new`

```rust pub fn new(builder: RawBrandedStructSchema) -> StructSchema { // ... // UNSAFE: access encoded nodes without validation } ```

This vulnerability allows safe Rust code to trigger UB, which violates Rust's safety guarantees.

The issue is resolved in version `0.24.0` by making constructor functions unsafe and mark the fields of struct as visible only in the crate.