USB: add pure specifiers and emit vtable

Issue #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`).

Author: ardnew

Diff for: cores/arduino/USB/PluggableUSBDevice.cpp

@@ -60,6 +60,10 @@ uint32_t arduino::internal::PluggableUSBModule::write_finish(usb_ep_t endpoint)
     return PluggableUSBD().write_finish(endpoint);
 }
 
+void arduino::internal::PluggableUSBModule::callback_reset()
+{
+}
+
 arduino::PluggableUSBDevice::PluggableUSBDevice(uint16_t vendor_id, uint16_t product_id)
     : USBDevice(get_usb_phy(), vendor_id, product_id, 1 << 8)
 {

Diff for: cores/arduino/USB/PluggableUSBDevice.h

@@ -38,6 +38,8 @@ class PluggableUSBModule {
     PluggableUSBModule(uint8_t numIfs) :
         numInterfaces(numIfs)
     { }
+    virtual ~PluggableUSBModule() 
+    { }
     void lock();
     void unlock();
     void assert_locked();
@@ -49,17 +51,25 @@ class PluggableUSBModule {
     uint32_t write_finish(usb_ep_t endpoint);
 
 protected:
-    virtual const uint8_t *configuration_desc(uint8_t index);
-    virtual void callback_reset() {};
-    virtual void callback_state_change(USBDevice::DeviceState new_state);
-    virtual uint32_t callback_request(const USBDevice::setup_packet_t *setup, USBDevice::RequestResult *result, uint8_t** data);
-    virtual bool callback_request_xfer_done(const USBDevice::setup_packet_t *setup, bool aborted);
-    virtual bool callback_set_configuration(uint8_t configuration);
-    virtual void callback_set_interface(uint16_t interface, uint8_t alternate);
-    virtual void init(EndpointResolver& resolver);
-    virtual const uint8_t *string_iinterface_desc();
+    virtual const uint8_t *configuration_desc(uint8_t index) = 0;
+    virtual void callback_state_change(USBDevice::DeviceState new_state) = 0;
+    virtual uint32_t callback_request(const USBDevice::setup_packet_t *setup, USBDevice::RequestResult *result, uint8_t** data) = 0;
+    virtual bool callback_request_xfer_done(const USBDevice::setup_packet_t *setup, bool aborted) = 0;
+    virtual bool callback_set_configuration(uint8_t configuration) = 0;
+    virtual void callback_set_interface(uint16_t interface, uint8_t alternate) = 0;
+    virtual void init(EndpointResolver& resolver) = 0;
+    virtual const uint8_t *string_iinterface_desc() = 0;
     virtual uint8_t getProductVersion() = 0;
 
+    /**
+     * Non-pure virtual method with a non-inline definition
+     *
+     * @note This satisfies all criteria necessary for the compiler to emit a
+     * vtable, allowing users to safely subclass any of the derived "pluggable"
+     * USB device classes (USBCDC, USBHID, USBMIDI, etc).
+     */
+    virtual void callback_reset();
+
     uint8_t pluggedInterface;
 
     const uint8_t numInterfaces;