Impl Distribution<u8> for Alphanumeric
Impl Distribution<u8> for Alphanumeric

Sampling a random alphanumeric string by collecting chars (that are known to be ASCII) into a String involves re-allocation as String is encoding to UTF-8, via the example:

```rust
let chars: String = iter::repeat(())
        .map(|()| rng.sample(Alphanumeric))
        .take(7)
        .collect();
```

I wanted to get rid of the clearly unnecessary re-allocations in my applications, so I needed to be able to access to the ASCII characters as simple bytes. It seems like that was already what was going on inside Alphanumeric however, it was just internal.

This PR changes the `Distribution<char>` impl to provide `u8`s (which it generates internally) instead, and implements the previous `Distribution<char>` using it. One could then, for example, do this:

```rust
let mut rng = thread_rng();
let bytes = (0..7).map(|_| rng.sample(ByteAlphanumeric)).collect();
let chars = unsafe { String::from_utf8_unchecked(bytes) };
```
rust-lang/rand