[RFA 00/12] (somewhat) clean up struct value ownership

[Tom Tromey <tom@tromey.com>]

struct value has long suffered from a complicated approach to
ownership.  In particular, values are reference counted, but they are
also handled specially if they are on the "value chain".  I believe
this has led to bugs on occasion; and anyway requires oddities like
release_value_or_incref.

This series goes some way toward cleaning up ownership for values.  It
introduces a gdb_ref_ptr specialization for values and then changes
various things to use it.  After this, it cleans up various code doing
manual memory mangement related to value; and in particular unifies
the value chain with the reference counting mechanism.

There is still more work that could be done in this area.  For
example:

* struct lval_funcs could be turned into a base class and then the
  implementers rewritten as ordinary objects.

* Likewise struct internalvar_funcs; and this would allow better type
  safety through the removal of union internalvar_data.

* Perhaps the "location" union could be removed from struct value,
  also providing more type safety.

However, I didn't do these, as this series seemed to reach a
reasonable stopping point.

Regression tested by the buildbot.

Tom