Create an object oriented interface for Hash Context

I liked the idea, recently [mentioned](http://marc.info/?l=php-internals&m=139301329924031&w=2) on internals, to remove the use of resources on use object oriented APIs instead.

This pull request does just that, by introducing the class ``HashContext``. It's targeted at master, since this should go into PHP 5.7.

Tests all pass, one required modification as it relied on the output of var_dump. The B/C break is very small, code that relies on checking ``is_resource()`` fails and needs to be changed to also do `ìnstanceof HashContext``.

A quick search on GitHub revealed no project that uses HashContext in the global namespace.