Can’t move out of BString using `into` methods

Because `ByteVec` methods operate through deref on `BString`, it is impossible to use the ones with move semantics (`into_string` etc.) to move out of a `BString`, which is confusing.

```rust
use bstr::{BString, ByteVec};

fn main() {
let x = BString::from("hello world");
x.into_string_lossy(); // cannot move out of dereference of `BString`
}
```

Start by reading the BString and ByteVec definitions and the into_* methods involved in the example. Determine how the move-based methods are exposed through deref and what API behavior is intended. Done means the reported BString move operation works without the dereference error, with regression coverage for the example.

Written by the indexing model from the issue text.