Add a basic hashtable data structure, with tests!

The basic hashtable is intended to be used to support a variety
of different datastructures such as map, set, multimap,
multiset, linkedmap, generationcache, etc.

Consequently its interface is fairly primitive.

The basic hashtable supports copy-on-write style functionality
using SharedBuffer.

The change introduces a simple generic function in TypeHelpers for
specifying hash functions.  The idea is to add template
specializations of hash_type<T> next to the relevant data structures
such as String8, String16, sp<T>, etc.

Change-Id: I2c479229e9d4527b4fbfe3b8b04776a2fd32c973