Merge HashTable and zend_array into a single data structure.

Hi, Please take a look into the patch https://github.com/php/php-src/pull/970/files This real changes are in zend_types.h, the rest is renaming that in most cases makes code cleaner. zend_array didn't change its binary representation, but now it's not possible to get a pointer to embedded HashTable. The same zend_array shoukd be used instead. Each HashTable got an extra 64-bit zend_refcounted header. This leads to some increase in memory consumption. The performance is slightly increased (may be measured with callgrind). The patch beaks one test (tests/lang/foreachLoopObjects.006.phpt), but actually it just disclose a problem that we have anyway. The patch should be a base for the future optimizations. e.g. removing HashTable->arData and/or HashTable->arHash and allocating them together with zend_array; introducing EG(empty_array) etc. Opinions are welcome... Thanks. Dmitry.