Commits on Nov 23, 2023

1. USB: add pure specifiers and emit vtable …

   Issue arduino#130 correctly identifies a newly-added method as pure virtual and
   fixes the declaration. However, for some reason it didn't address all of
   the other virtual methods in that same class (`PluggableUSBModule`) that
   do not define a default implementation.
   
   The only virtual method that has a default implementation is provided
   inline in the class interface:
   
   ```c++
       virtual void callback_reset() {};
   ```
   
   These issues combined prevent the compiler from being able to emit a
   vtable for the `PluggableUSBModule` class, thus preventing users from
   correctly subclassing it or any one of its derived classes such as
   `USBCDC`, `USBHID`, `USBMIDI`, etc. Refer to the following answer on
   StackOverflow for a detailed explanation of the issue:
   
   https://stackoverflow.com/a/57504289/1054397
   
   This PR adds the pure specifier (`= 0`) to all of the virtual methods in
   this class that do not have a default implementation. It also moves the
   default empty definition of `virtual void callback_reset()` to the class
   definition in `USB/PluggableUSBDevice.cpp` so that this class complies
   completely with the criteria for emitting a vtable.
   
   > #### Note
   >
   > The error that I was encountering prior to these changes was pretty
   > cryptic (from PlatformIO):
   >
   > ```txt
   > .pio/build/hid/src/target.cpp.o: In function `foo()':
   > USBHID/src/PluggableUSBHID.h:53: undefined reference to
   >     `vtable for arduino::internal::PluggableUSBModule'
   > .pio/build/hid/src/target.cpp.o: In function `foo()':
   > foo.hpp:100: undefined reference to
   >     `vtable for arduino::internal::PluggableUSBModule'
   > collect2: error: ld returned 1 exit status
   > *** [.pio/build/hid/firmware.elf] Error 1
   > ```
   >
   > Even stranger, the error would only be generated with a debug build;
   > i.e., the only difference in command-line arguments was the additional
   > CFLAGS of `-Og -g2 -ggdb2`. Without the debug flags, my project was
   > building without error.
   >
   > With the changes in this PR, my project now builds with and without
   > these additional debug flags. Further verification was performed by
   > testing the example sketches `Keyboard`, `KeyboardRaw`, and `Mouse`
   > from library `USBHID` as well as using the core `Serial` object for
   > ordinary USB serial I/O (`USBCDC`).

   

   ardnew committed Nov 23, 2023
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