Improved interface to the Mem object handling.  Small size reduction and
performance increase.
In the Mem object, stop requiring that Mem.xDel be NULL when the MEM_Dyn
bit is clear.  Also reduce the amount of initialization of Mem objects.
All for a small size reduction and performance increase.
Performance improvement and slight size reduction to the comparison operators
in the VDBE.
Fix compiler warnings and change the nullMem structure initializer into a
format that MSVC can understand.
Merge micro-optimizations into trunk after fixing the build on MSVC.
Performance now shows 7.58% faster than the 3.8.6 release on x64 with
gcc 4.8.1 and -Os.
Make sure of the strchrnul() library function on platforms where it is
available.
Fix harmless warnings on 32-bit MSVC builds.