Dynamic erasure (#4653)

Remove erasure-related constants

Remove unneeded `Iterator::collect` call

Document erasure module

Randomize coding blobs used for repair

core/src/chacha.rs

@@ -133,7 +133,7 @@ mod tests {
         hasher.hash(&buf[..size]);
 
         //  golden needs to be updated if blob stuff changes....
-        let golden: Hash = "E2HZjSC6VgH4nmEiTbMDATTeBcFjwSYz7QYvU7doGNhD"
+        let golden: Hash = "AMKCEbK6txetPPEQ8JDVdrqpefgtGRzN6ng2gSmKy6Fv"
             .parse()
             .unwrap();