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/cmp.rs

@@ -83,10 +83,8 @@ fn eq_test() {
 mod benches {
     use test::Bencher;
 
-    use sha256;
-    use sha512;
-    use Hash;
-    use cmp::fixed_time_eq;
+    use crate::{Hash, sha256, sha512};
+    use crate::cmp::fixed_time_eq;
 
     #[bench]
     fn bench_32b_constant_time_cmp_ne(bh: &mut Bencher) {

src/hmac.rs

@@ -390,8 +390,7 @@ mod tests {
 mod benches {
     use test::Bencher;
 
-    use sha256;
-    use {Hmac, Hash, HashEngine};
+    use crate::{Hmac, Hash, HashEngine, sha256};
 
     #[bench]
     pub fn hmac_sha256_10(bh: & mut Bencher) {

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) {
@@ -273,9 +274,7 @@ mod tests {
 mod benches {
     use test::Bencher;
 
-    use sha1;
-    use Hash;
-    use HashEngine;
+    use crate::{Hash, HashEngine, sha1};
 
     #[bench]
     pub fn sha1_10(bh: & mut Bencher) {

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.
     ///
@@ -523,9 +524,7 @@ mod tests {
 mod benches {
     use test::Bencher;
 
-    use sha256;
-    use Hash;
-    use HashEngine;
+    use crate::{Hash, HashEngine, sha256};
 
     #[bench]
     pub fn sha256_10(bh: & mut Bencher) {

src/sha256d.rs

@@ -153,7 +153,7 @@ input: &'static str,
 mod benches {
     use test::Bencher;
 
-    use crate::{HashEngine, sha256d};
+    use crate::{Hash, HashEngine, sha256d};
 
     #[bench]
     pub fn sha256d_10(bh: & mut Bencher) {

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) {
@@ -442,9 +443,7 @@ mod tests {
 mod benches {
     use test::Bencher;
 
-    use sha512;
-    use Hash;
-    use HashEngine;
+    use crate::{Hash, HashEngine, sha512};
 
     #[bench]
     pub fn sha512_10(bh: & mut Bencher) {