NatVis files describe how to display types in some Windows debuggers,
such as Visual Studio, WinDbg, and VS Code.
This commit makes several improvements:
* Adds visualizers for Rc<T>, Weak<T>, and Arc<T>.
* Changes [size] to [len], for consistency with the Rust API.
Visualizers often use [size] to mirror the size() method on C++ STL
collections.
* Several visualizers used the PVOID and ULONG typedefs. These are part
of the Windows API; they are not guaranteed to always be defined in a
pure Rust DLL/EXE. I converted PVOID to `void*` and `ULONG` to
`unsigned long`.
* Cosmetic change: Removed {} braces around the visualized display
for `Option` types. They now display simply as `Some(value)` or
`None`, which reflects what is written in source code.
* The visualizer for `alloc::string::String` makes assumptions about
the layout of `String` (it casts `String*` to another type), rather
than using symbolic expressions. This commit changes the visualizer
so that it simply uses symbolic expressions to access the string
data and string length.