[v3] hashtable aliases

[Benjamin De Kosnik]

This patch restructures the std::unordered_* containers, as per the
tempting comment in unordered_set:

  // NB: When we get typedef templates these class definitions
  // will be unnecessary.

This required some changes in _Hashtable, forced the consolidation of
insert() logic in a base class, and necessitated the restructuring of
type definitions in the _Hashtable. I think the current form allows
some nice advantages in terms of nested types and expressiveness. 

tested x86/linux
tested x86/linux -debug

-benjamin
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