impl Clone for Digest

I needed a way to feed a Digest a stream of bytes while checking the CRC
for possible matches after each byte. Using CRC::crc directly would make
this O(n^2), so I looked to Digest. Because Digest::finalize consumes
self, it must be thrown out after each finalize operation. This is
probably what you want in the general case, but not in my slightly weird
case.

This problem can be solved by allowing Digest to be duplicated, thereby
letting the user "fork" the CRC state as desired. (This also enables
certain niche CRC-search algorithms.)

I went with Clone rather than Copy because making Digest Copy seemed
potentially error-prone; Clone has the advantage of requiring explicit
intent.

src/lib.rs

@@ -46,6 +46,7 @@ pub struct Crc<W: Width> {
     table: [W; 256],
 }
 
+#[derive(Clone)]
 pub struct Digest<'a, W: Width> {
     crc: &'a Crc<W>,
     value: W,