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) };
```

Author: qoh

src/distributions/other.rs

@@ -19,7 +19,7 @@ use serde::{Serialize, Deserialize};
 
 // ----- Sampling distributions -----
 
-/// Sample a `char`, uniformly distributed over ASCII letters and numbers:
+/// Sample a `char` or `u8`, uniformly distributed over ASCII letters and numbers:
 /// a-z, A-Z and 0-9.
 ///
 /// # Example
@@ -83,6 +83,13 @@ impl Distribution<char> for Standard {
 
 impl Distribution<char> for Alphanumeric {
     fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> char {
+        let byte: u8 = self.sample(rng);
+        byte as char
+    }
+}
+
+impl Distribution<u8> for Alphanumeric {
+    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> u8 {
         const RANGE: u32 = 26 + 26 + 10;
         const GEN_ASCII_STR_CHARSET: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ\
                 abcdefghijklmnopqrstuvwxyz\
@@ -94,7 +101,7 @@ impl Distribution<char> for Alphanumeric {
         loop {
             let var = rng.next_u32() >> (32 - 6);
             if var < RANGE {
-                return GEN_ASCII_STR_CHARSET[var as usize] as char;
+                return GEN_ASCII_STR_CHARSET[var as usize];
             }
         }
     }