Move Hashers to util/hasher.{cpp/h}

Move the hashers that we use for hash tables to a common place.

Moved hashers:
- SaltedTxidHasher
- SaltedOutpointHasher
- FilterHeaderHasher
- SignatureCacheHasher
- BlockHasher

src/index/blockfilterindex.h

@@ -9,15 +9,11 @@
 #include <chain.h>
 #include <flatfile.h>
 #include <index/base.h>
+#include <util/hasher.h>
 
 /** Interval between compact filter checkpoints. See BIP 157. */
 static constexpr int CFCHECKPT_INTERVAL = 1000;
 
-struct FilterHeaderHasher
-{
-    size_t operator()(const uint256& hash) const { return ReadLE64(hash.begin()); }
-};
-
 /**
  * BlockFilterIndex is used to store and retrieve block filters, hashes, and headers for a range of
  * blocks by height. An index is constructed for each supported filter type with its own database