`HashMap` and `HashSet` should provide a getter for their hashers

``` rust
impl<K: Eq + Hash, V, S: BuildHasher> HashMap<K, V, S> {
    pub fn hasher(&self) -> &S { &self.hash_builder }
}

impl<T: Eq + Hash, S: BuildHasher> HashSet<T, S> {
    pub fn hasher(&self) -> &S { self.map.hasher() }
}
```

This would let us close contain-rs/linked-hash-map#3, and likely has other (niche) uses for code that needs to reproduce hashes.
