[RFA 00/20] more use of C++ in the Python layer

[Tom Tromey <tom@tromey.com>]

This patch series removes many more cleanups from the Python layer.
It mostly replaces ensure_python_env, but also fixes a few spots that
were missing in the earlier series.  You may also want to take notice
of the htab_up patch, which adds a unique_ptr typedef for managing
libiberty hash tables.

These patches require my earlier gdbpy_reference series.

I split this series up quite a bit to ease review.  The earlier
patches are all relatively straightforward translations.  Later
patches involve multiple changes in a function: if a function used
ensure_python_env and also had other cleanups (specifically
Python-related ones), I've tried to transform the entire function at
once, to avoid any ordering possible issues.

A few uses of ensure_python_env remain after this series.  I will
address them later.  My end goal is to remove all cleanups from the
Python layer, and ideally all the goto-based control flow as well;
this isn't totally achievable until much of the rest of gdb has moved
away from cleanups, but it's certainly possible to get close.

I've built and tested this locally.  I will run it through the
buildbot as well -- long story, but I have to wait until one upstream
patch lands to reset the baseline before I can do this.

Tom