Remove redundant field names

Now we have edition 2018 clippy emits warning:

  warning: redundant field names in struct initialization

As suggested, remove the redundant field names.

src/hmac.rs

@@ -100,8 +100,8 @@ impl<T: Hash> HmacEngine<T> {
     /// "inner" and "outer" engines.
     pub fn from_inner_engines(iengine: T::Engine, oengine: T::Engine) -> HmacEngine<T> {
         HmacEngine {
-            iengine: iengine,
-            oengine: oengine,
+            iengine,
+            oengine,
         }
     }
 }

src/sha256.rs

@@ -246,7 +246,7 @@ impl HashEngine {
         HashEngine {
             buffer: [0; BLOCK_SIZE],
             h: ret,
-            length: length,
+            length,
         }
     }
 

src/siphash24.rs

@@ -91,8 +91,8 @@ impl HashEngine {
     /// Create a new SipHash24 engine with keys.
     pub fn with_keys(k0: u64, k1: u64) -> HashEngine {
         HashEngine {
-            k0: k0,
-            k1: k1,
+            k0,
+            k1,
             length: 0,
             state: State {
                 v0: k0 ^ 0x736f6d6570736575,