Expose memory usage

Currently `Regex` is this struct:

```rust
pub struct Regex {
    pub(crate) meta: meta::Regex,
    pub(crate) pattern: Arc<str>,
}
```

Most of the methods proxy to `self.meta` methods. One notable exception is `memory_usage`, which exists in `meta::Regex`, but is missing on the public struct. It would be nice to have it for accounting purposes.