Remove the C macro version of array_elements.
Move the C++ template version of array_elements to template_utils.h

Remove usage of DBUG macros from template_utils.h, use the standard
assert() and -DNDEBUG instead.

Remove macros C_MODE_START and C_MODE_END
Most usage was obsolete, all code has been converted from C to C++
Some usage is still needed: in plugins, so use 'extern "C"' instead.

Change-Id: I2b81a6f07cf2e6a56078f4e716e2bdf96510d199