Improve gdbmi_pt.h documentation.
Cleanup: remove gdbmi_oc until needed.
Reducing complexity of gdbmi_pt.h

Create gdbmi_pt_alloc.h which will most likely be a permanent file
in the gdbmi repository.

The file gdbmi_pt_print.h is used temporary for unit testing but
I think I will remove it shortly in favor of unit testing the
data gdbmi parse tree with in memory assertions.

Printing the data structure will be best done in python.