cuckoocache: Return approximate memory size

Returning the approximate total size eliminates the need for
InitS*Cache() to do nElems*sizeof(uint256). The cuckoocache has a better
idea of this information.

src/script/sigcache.h

@@ -33,6 +33,6 @@ public:
     bool VerifySchnorrSignature(Span<const unsigned char> sig, const XOnlyPubKey& pubkey, const uint256& sighash) const override;
 };
 
-void InitSignatureCache();
+[[nodiscard]] bool InitSignatureCache();
 
 #endif // BITCOIN_SCRIPT_SIGCACHE_H