initial rtmidi support

[artfwo]

first steps for rtmidi-based midi implementation:

• rtmidi import (needs submodules update)
• link isms with rtmidi and libasound (on linux, other platforms to go)
• single rtmidi virtual port for in and out
• stubs for incoming message callback and send function

TODO:

• bridging midi IN to clock and lua interface (message callback).
• bridging lua interface to midi out.
• build rtmidi with JACK support, maybe with a command-line switch to isms to choose midi API
• build rtmidi with appropriate backends on win32 and macos
• explore port selection and management in the lua domain.

[tehn]

this is all a great start!