Add a hash-index component for block

Summary:
this is the key component extracted from diff: https://reviews.facebook.net/D14271
I separate it to a dedicated patch to make the review easier.

Test Plan: added a unit test and passed it.

Reviewers: haobo, sdong, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16245

util/arena.h

@@ -39,12 +39,12 @@ class Arena {
   // Returns an estimate of the total memory usage of data allocated
   // by the arena (exclude the space allocated but not yet used for future
   // allocations).
-  const size_t ApproximateMemoryUsage() {
+  size_t ApproximateMemoryUsage() const {
     return blocks_memory_ + blocks_.capacity() * sizeof(char*) -
            alloc_bytes_remaining_;
   }
 
-  const size_t MemoryAllocatedBytes() { return blocks_memory_; }
+  size_t MemoryAllocatedBytes() const { return blocks_memory_; }
 
  private:
   // Number of bytes allocated in one block

util/hash.cc

@@ -46,5 +46,4 @@ uint32_t Hash(const char* data, size_t n, uint32_t seed) {
   return h;
 }
 
-
 }  // namespace rocksdb

util/murmurhash.h

@@ -11,6 +11,7 @@
 */
 #pragma once
 #include <stdint.h>
+#include "rocksdb/slice.h"
 
 #if defined(__x86_64__)
 #define MURMUR_HASH MurmurHash64A
@@ -29,5 +30,13 @@ typedef unsigned int murmur_t;
 unsigned int MurmurHashNeutral2 ( const void * key, int len, unsigned int seed );
 #define MurmurHash MurmurHashNeutral2
 typedef unsigned int murmur_t;
-
 #endif
+
+// Allow slice to be hashable by murmur hash.
+namespace rocksdb {
+struct murmur_hash {
+  size_t operator()(const Slice& slice) const {
+    return MurmurHash(slice.data(), slice.size(), 0);
+  }
+};
+}  // rocksdb