[map] Speedup for int types

Since our int hash is now good, we don't need the modulo prime
here, so forgo it.  Major speedup:

Comparing before to after
Benchmark                              Time             CPU      Time Old      Time New       CPU Old       CPU New
-------------------------------------------------------------------------------------------------------------------
BM_MapInsert/16                     -0.4136         -0.4137             6             4             6             4
BM_MapInsert/64                     -0.4442         -0.4441             6             3             6             3
BM_MapInsert/512                    -0.5382         -0.5383             8             4             8             4
BM_MapInsert/4096                   -0.4160         -0.4162             8             5             8             5
BM_MapInsert/32768                  -0.3256         -0.3258            12             8            12             8
BM_MapInsert/262144                 -0.1723         -0.1727            11            10            11             9
BM_MapInsert/1048576                -0.2310         -0.2309            28            22            28            22
BM_MapLookup/16                     -0.0247         -0.0247             3             3             3             3
BM_MapLookup/64                     -0.1039         -0.1038             3             3             3             3
BM_MapLookup/512                    -0.1076         -0.1079             4             3             4             3
BM_MapLookup/4096                   -0.3729         -0.3732             9             6             9             6
BM_MapLookup/32768                  +0.2467         +0.2468             9            12             9            12
BM_MapLookup/262144                 -0.1862         -0.1868            14            11            14            11
BM_MapLookup/1048576                +0.1159         +0.1160            15            17            15            17
OVERALL_GEOMEAN                     -0.2414         -0.2416             0             0             0             0

src/hb-map.hh

@@ -212,12 +212,12 @@ struct hb_hashmap_t
 
     hash &= 0x3FFFFFFF; // We only store lower 30bit of hash
     unsigned int tombstone = (unsigned int) -1;
-    unsigned int i = hash % prime;
+    unsigned int i = std::is_integral<K>::value ? hash & mask : hash % prime;
     unsigned length = 0;
     unsigned step = 0;
     while (items[i].is_used ())
     {
-      if ((hb_is_same (K, hb_codepoint_t) || items[i].hash == hash) &&
+      if ((std::is_integral<K>::value || items[i].hash == hash) &&
 	  items[i] == key)
         break;
       if (items[i].is_tombstone () && tombstone == (unsigned) -1)
@@ -295,11 +295,11 @@ struct hb_hashmap_t
     if (unlikely (!items)) return nullptr;
 
     hash &= 0x3FFFFFFF; // We only store lower 30bit of hash
-    unsigned int i = hash % prime;
+    unsigned int i = std::is_integral<K>::value ? hash & mask : hash % prime;
     unsigned step = 0;
     while (items[i].is_used ())
     {
-      if ((hb_is_same (K, hb_codepoint_t) || items[i].hash == hash) &&
+      if ((std::is_integral<K>::value || items[i].hash == hash) &&
 	  items[i] == key)
       {
 	if (items[i].is_real ())