Allow many single character names

When implementing `HashEngine` we use a bunch of local variables that
are bytes within the hash. These are sequential so usage of a,b,c,... is
perfectly readable and expressive.

Shoosh clippy.

src/sha1.rs

@@ -137,6 +137,7 @@ impl crate::Hash for Hash {
     }
 }
 
+#[allow(clippy::many_single_char_names)]
 impl HashEngine {
     // Basic unoptimized algorithm from Wikipedia
     fn process_block(&mut self) {

src/sha256.rs

@@ -230,6 +230,7 @@ macro_rules! round(
     )
 );
 
+#[allow(clippy::many_single_char_names)]
 impl HashEngine {
     /// Create a new [HashEngine] from a midstate.
     ///

src/sha512.rs

@@ -217,6 +217,7 @@ macro_rules! round(
     )
 );
 
+#[allow(clippy::many_single_char_names)]
 impl HashEngine {
     // Algorithm copied from libsecp256k1
     fn process_block(&mut self) {