Some small changes to able to build hhvm

We are now investigating HipHop, but for that we need to port our existing PHP extensions. I had to change two small things in hiphop code, to be able to compile it together with one of our extensions. I think these changes could be useful not only for us.

1) Renamed macro "field_type" to "hphp_field_type", because it was making conflicts with mysqlpp library, which have public method "field_type" in some class. It is pretty generic name, so I believe, it could make also problems with other third party libraries in future.

2) At some moment I started to get error messages "error: ‘size’ is not a member template function" in hiphop, not our code. Don't know exactly why they started to appear, but after some googling I found (http://stackoverflow.com/questions/10671406/c-confusing-attribute-name-for-member-template) that this is known GCC bug and workaround is very simple - put parenthesis around "old->size" in hphp/runtime/vm/tread-hash-map.h:203.