Use "ropes" for string concatenation

Hi, Recently, Xinchen and me worked on optimization that eliminates useless reallocations and copying during string concatenation (ZEND_ADD_STRING and family + ZEND_CONCAT). The idea comes from ropes, but adopted especially for our needs. Rope is popular data structure in languages with immutable strings. http://en.wikipedia.org/wiki/Rope_%28data_structure%29 We don't try to use ropes everywhere in the engine (at least it's too later for 7.0), only for concatenation. The first branch uses ropes only instead of ZEND_ADD_STRING and family. This must be safe. The only problem is possible memory leaks on exception (but we already have this problem anyway). The simplest way to understand the patch - read code for new opcodes in zend_vm_def.h. https://github.com/php/php-src/pull/1194/files The second branch in addition uses ropes for series of ZEND_CONCAT. It disables calls to do_operation(ZEND_CONCAT) handler of custom internal classes. https://github.com/php/php-src/pull/1195/files Both make slight speed improvement (first +0.3%, second +0.6% on wordpress home page). We don't currently use ability to override CONCAT behavior in internal classes, and I'm not sure if it may be useful at all. (For example Lua doesn't provide concat meta-method). May be remove it? Thoughts and comments are welcome. Thanks. Dmitry.