gvpack: replace CDT attribute dictionaries with std::maps

gvpack: replace CDT attribute dictionaries with std::maps

This is intended to be a non-functional change with a number of benefits:

  1. This is C++ code, so no need for custom CDT data structures. The STL
     equivalents are likely more optimized, better understood by the compiler,
     and more familiar to human readers.

  2. We avoid the lib/common allocation routines (`NEW` macro and friends). Now
     allocation failures naturally propagate outwards as `std::bad_alloc`
     exceptions without any custom handling.

  3. Related to (2), we no longer need to manually free any of these objects.
     RAII takes care of all of this.