* resolves FortySevenEffects#40 with franky47's proposed thru filter overhaul
* removes thru filter modes
* adds thru filter callback
* adds thru map callback
* old thru filter unit tests have been replicated with filter callbacks
* does not yet include documentation changes

I believe this implements the latest proposal for FortySevenEffects#40 and exercises
everything necessary in the unit tests, including the immutability of
`mMessage` after a thru map callback has modified the outgoing message.

The thru filter callbacks in the unit tests are not suitable for copying
and pasting by end-users due to the difference in the MIDI namespace
when setup via the unit tests vs via `MIDI_CREATE_DEFAULT_INSTANCE()`.

If the changes here are deemed suitable, I'll work on documentation.

This is ground work for the `map` Thru function
in PR FortySevenEffects#232.

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:
FortySevenEffects#232 (comment)