Added Iterable, a class that vends standard C++
iterators for LLDB's container data structures.
Iterable abstracts over the backing data structure,
ignoring keys for maps for example.  It also provides
locking as a service so that the code

for (ThreadSP thread_sp : process->Threads())
{
  // ... use thread_sp
}

takes the appropriate locks once, without having to
do anything else.

The salient advantages of this system are:

- Much simpler and idiomatic loop code
- Lock once instead of each time an element is fetched
- Less boilerplate to produce the iterators

The intent is that Iterable will replace Get...AtIndex
in most places, and that ForEach(), which solves the
same problem in a less-idiomatic way, be phased out in
favor of this approach.

I've added Iterables to ThreadList, TypeList, and
Process (which is really just forwarding to ThreadList).