[portmidi] Add overlay with iOS patches

[fwcd]

This vendors the portmidi port until PortMidi/portmidi#65 is merged. Note that this version of PortMidi is based on the latest master, which means we can also drop the other two patches.

[daschuer]

It looks like this touched the code for iOS and macOS, which introduces a risk of a regression I am not able to estimate. The upstream PR is also still pending.

What would be the best strategy for 2.5?
One option is a fast merge for a high test rate. The other option is postponing this to 2.6 or/and making this an iOS only patch.

What do you think?

[fwcd]

I'd consider it relatively safe to merge for 2.5. It does introduce a new abstraction, but the implementation for macOS stays exactly the same. Test-driving it during the beta phase would make sure nothing slipped through.

(Also a reason that the diff is so large is that it includes the migration to <stdint.h> types, i.e. uint64_t instead of UInt64. We've discussed that in the upstream review and since the PR introduces some new Apple-specific code, the decision was that it was easiest to include that change in the PR: PortMidi/portmidi#65 (comment))

[fwcd]

Would be fine with 2.6 too though, it's your call

[daschuer]

LGTM, OK than let's go for it.