feat: Export type definitions when using macros

Types have names prepended by the port name
(defaults to `MIDI`), to allow multi-port applications.

This allows referencing those types elsewhere in the
application, without re-writing the template arguments,
and simplifies referencing the underlying Message type,
for SoftThru `filter`/`map`function definitions.

Proposition & discussion:
#232 (comment)

examples/ThruFilterMap/ThruFilterMap.ino

@@ -1,7 +1,5 @@
 #include <MIDI.h>
 
-using Message = midi::Message<midi::DefaultSettings::SysExMaxSize>;
-
 MIDI_CREATE_DEFAULT_INSTANCE();
 
 /**
@@ -17,7 +15,7 @@ MIDI_CREATE_DEFAULT_INSTANCE();
  * allowing to use a keyboard to change patches on a MIDI device.
  */
 
-bool filter(const Message& message)
+bool filter(const MIDIMessage& message)
 {
     if (message.type == midi::NoteOn)
     {
@@ -27,10 +25,10 @@ bool filter(const Message& message)
     return false;
 }
 
-Message map(const Message& message)
+MIDIMessage map(const MIDIMessage& message)
 {
     // Make a copy of the message
-    Message output(message);
+    MIDIMessage output(message);
     if (message.type == midi::NoteOn)
     {
         output.type = midi::ProgramChange;

src/serialMIDI.h

@@ -52,7 +52,7 @@ class SerialMIDI
 
 public:
     static const bool thruActivated = true;
-    
+
     void begin()
 	{
         // Initialise the Serial port
@@ -103,9 +103,12 @@ END_MIDI_NAMESPACE
  Example: MIDI_CREATE_INSTANCE(HardwareSerial, Serial2, midi2);
  Then call midi2.begin(), midi2.read() etc..
  */
-#define MIDI_CREATE_INSTANCE(Type, SerialPort, Name)  \
-    MIDI_NAMESPACE::SerialMIDI<Type> serial##Name(SerialPort);\
-    MIDI_NAMESPACE::MidiInterface<MIDI_NAMESPACE::SerialMIDI<Type>> Name((MIDI_NAMESPACE::SerialMIDI<Type>&)serial##Name);
+#define MIDI_CREATE_INSTANCE(Type, SerialPort, Name) \
+    using Name##SerialTransport = MIDI_NAMESPACE::SerialMIDI<Type>; \
+    using Name##Interface = MIDI_NAMESPACE::MidiInterface<Name##SerialTransport>; \
+    using Name##Message = Name##Interface::MidiMessage; \
+    Name##SerialTransport serial##Name(SerialPort); \
+    Name##Interface Name((Name##SerialTransport&)serial##Name);
 
 #if defined(ARDUINO_SAM_DUE) || defined(USBCON) || defined(__MK20DX128__) || defined(__MK20DX256__) || defined(__MKL26Z64__)
     // Leonardo, Due and other USB boards use Serial1 by default.
@@ -125,6 +128,9 @@ END_MIDI_NAMESPACE
  @see DefaultSettings
  @see MIDI_CREATE_INSTANCE
  */
-#define MIDI_CREATE_CUSTOM_INSTANCE(Type, SerialPort, Name, Settings)           \
-    MIDI_NAMESPACE::SerialMIDI<Type> serial##Name(SerialPort);\
-    MIDI_NAMESPACE::MidiInterface<MIDI_NAMESPACE::SerialMIDI<Type>, Settings> Name((MIDI_NAMESPACE::SerialMIDI<Type>&)serial##Name);
+#define MIDI_CREATE_CUSTOM_INSTANCE(Type, SerialPort, Name, Settings) \
+    using Name##SerialTransport = MIDI_NAMESPACE::SerialMIDI<Type>; \
+    using Name##Interface = MIDI_NAMESPACE::MidiInterface<Name##SerialTransport, Settings>; \
+    using Name##Message = Name##Interface::MidiMessage; \
+    Name##SerialTransport serial##Name(SerialPort); \
+    Name##Interface Name((Name##SerialTransport&)serial##Name);