Data streams

I see the `estimate()` function is limited to a vector of `String`, but the CVM algorithm is a lot more powerful than that.

Why not a function signature like this?

```rust
fn estimate(source: impl IntoIterator, delta: f64, epsilon: f64) -> usize
where
T: PartialOrd + PartialEq,
```

Or a structure?

```rust
let mut estimator = Estimator::::new();

estimator.push("hello".to_string());
estimator.push("bye".to_string());
estimator.push("hello".to_string());

let num_words = estimator.estimate();

assert_eq!(num_words, 2);
```