Reorganize xml_memory_page structure

The page no longer contains 'data' field to use sizeof everywhere instead
of offsetof/sizeof inconsistency (that is required because some compilers
don't recognize offsetof as compile-time constant).

The page no longer contains 'memory' field that is now encoded as an
offset byte before the page - this allows us to save one pointer from the
static page in the document to keep the size the same as in v1.2 (binary
compatibility).

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1046 99668b35-9821-0410-8761-19e4c4f06640