Fix clippy warnings

Clippy default settings seemed to have changed introducing a few new
warnings.

warning: variable does not need to be mutable
warning: deref on an immutable reference
warning: returning the result of a `let` binding from a block

Fix them all in a single patch because CI has to pass for each patch.

Author: tcharding

src/context.rs

@@ -189,6 +189,8 @@ mod alloc_only {
         /// # }
         /// ```
         #[cfg_attr(not(feature = "rand-std"), allow(clippy::let_and_return, unused_mut))]
+        #[allow(unused_mut)] // ctx is not mutated under some feature combinations.
+        #[allow(clippy::let_and_return)] // ctx as for unused_mut
         pub fn gen_new() -> Secp256k1<C> {
             #[cfg(target_arch = "wasm32")]
             ffi::types::sanity_checks_for_wasm();

src/ecdsa/serialized_signature.rs

@@ -45,7 +45,7 @@ impl PartialEq for SerializedSignature {
 impl AsRef<[u8]> for SerializedSignature {
     #[inline]
     fn as_ref(&self) -> &[u8] {
-        &*self
+        self
     }
 }