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.cpp

@@ -15,28 +15,6 @@
 #include <boost/thread.hpp>
 
 namespace {
-
-/**
- * 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;
-    }
-};
-
 /**
  * Valid signature cache, to avoid doing expensive ECDSA signature checking
  * twice for every transaction (once when accepted into memory pool, and