Explicitly delete registered converter chains

[kbluck]

While working with boost.python, I've noted that
instantiating most specializations of
boost::python::extract<> results in MSVC leak reports.
My usual tactic for third-party libraries in such cases
is to suppress the CRT tracking for the affected code.
This is how I deal with the abundant "leaks" from
Python due to not calling Py_Finalize(), for example.
In this case, though, the allocations are pre-main()
and I can't get a suppression hook in ahead of it.

I realize that these aren't true leaks, and that there
is some reluctance to ferret out such situations at the
moment due to the lack of support for Py_Finalize(),
but this generates a *lot* of leak reports and makes it
very difficult to recognize true leaks. It was actually
making me consider not using extract<> at all.

I realized that was an overreaction, so instead I added
a destructor for struct
boost::python::converter::registration that explicitly
walks and deletes the singly-linked list nodes
contained therein. This resolved my current flurry of
extract<>-related leak reports.

[david_abrahams]

Logged In: YES 
user_id=52572

Accepted, but changed to factor more code into the
Boost.Python compiled library rather than inlining it in
each extension module.

I do think these are "true leaks," actually.  Py_Finalize
support would not have touched this memory.  Thanks for the
patch.  It's in the CVS HEAD (too late for 1.34.0).