Deduplicate SignatureCacheHasher

This moves the SignatureCacheHasher to the sigcache header, out of the anonymous
namespace, so that the tests can import it.

src/script/sigcache.h

@@ -19,6 +19,27 @@ static const int64_t MAX_MAX_SIG_CACHE_SIZE = 16384;
 
 class CPubKey;
 
+/**
+ * We're hashing a nonce into the entries themselves, so we don't need extra
+ * blinding in the set hash computation.
+ *
+ * This may exhibit platform endian dependent behavior but because these are
+ * nonced hashes (random) and this state is only ever used locally it is safe.
+ * All that matters is local consistency.
+ */
+class SignatureCacheHasher
+{
+public:
+    template <uint8_t hash_select>
+    uint32_t operator()(const uint256& key) const
+    {
+        static_assert(hash_select <8, "SignatureCacheHasher only has 8 hashes available.");
+        uint32_t u;
+        std::memcpy(&u, key.begin()+4*hash_select, 4);
+        return u;
+    }
+};
+
 class CachingTransactionSignatureChecker : public TransactionSignatureChecker
 {
 private: