Intent to (start) Ship(ping) (sorta): WebMIDI

[Kyle Machulis]

After a few years of development as I've had time, the first portion of
WebMIDI (bug 1201590), the web standard API for accessing MIDI-enabled
hardware (this is not related to MIDI files or playback), should be landing
soon.

This bug only includes the core DOM API code and tests, with no actual
platform implementations yet. It will be pref'd off (dom.webmidi.enabled)
for the time being, as not that much can really be done with it until we
actually get some real platform support going and we don't want to confuse
nightly users with queries about bringing up the test MIDI platform/devices.

The plan now is to first implement Mac OS X support (bug 1201593), then
Windows support (bug 1201596, not sure if this will be Win32 or UWP based,
need to discuss that), and finally Linux support (bug 1201598).

Following my goal of only writing new system specific code in Rust, all of
these modules should be Rust based, talking to the C++ DOM implementation
(until such time that we can emit Rust from the IPDL parser, in which case
we can start oxidizing our way up). We've got a demo version of OS X
support in C++ already, so I'm hoping that the OS X implementation can
build on that as well as on the great Rust-based system work the WebAuthn
team did, and we can get that going soon. I'll also be talking to the Rust
community to see if there's any pre-existing crates we can leverage for
this.

Once we've actually finished a platform module and can use this with real
hardware, we can discuss how to ship this on wider platforms.

Here's to more music via the web!