Refactor SipHash_32b benchmark to improve accuracy and avoid optimization issues

- Modify `SipHash_32b` benchmark to use `FastRandomContext` for generating initial values.
- Cycle through and modify each byte of the `uint256` value to ensure no part of it can be optimized away.

The lack of "recursion" (where the method call overwrites the used inputs partially) and the systematic modification of each input byte makes the benchmark usage more reliable and thorough.

src/bench/crypto_hash.cpp

@@ -192,10 +192,16 @@ static void SHA512(benchmark::Bench& bench)
 
 static void SipHash_32b(benchmark::Bench& bench)
 {
-    uint256 x;
-    uint64_t k1 = 0;
+    FastRandomContext rng{/*fDeterministic=*/true};
+    auto k0{rng.rand64()}, k1{rng.rand64()};
+    auto val{rng.rand256()};
+    auto i{0U};
     bench.run([&] {
-        *((uint64_t*)x.begin()) = SipHashUint256(0, ++k1, x);
+        ankerl::nanobench::doNotOptimizeAway(SipHashUint256(k0, k1, val));
+        ++k0;
+        ++k1;
+        ++i;
+        val.data()[i % uint256::size()] ^= i & 0xFF;
     });
 }