Faster hashing

1000000 Hashes
```
SM Timing
-----------
SizeT hash 	      1	00.061367	(00.061367 +- 00.000000)	[00.061367,00.061367]
String hash	      1	01.651815	(01.651815 +- 00.000000)	[01.651815,01.651815]
```
A posteriori nit: Wouldn't it be better to use [enable_if](http://en.cppreference.com/w/cpp/types/enable_if) ?