Factor out the code for choosing the bucket count, and which bucket that hash values map to make it easier to experiment with alternative policies.

[SVN r44697]

libs/unordered

@@ -1 +1 @@
-Subproject commit ed97a43f40b8ccc229ea10f2bdd5f3cf9049c4ed
+Subproject commit c56213442d4836bd92fdaf52899fde28f63b996f