Retain cycle between MIKMIDISequence and MIKMIDISequencer

[rrc2soft]

As mentioned in the bug title, there is a retain cycle between those two classes. Therefore, if an instance of MIKMIDISequencer is set to nil, such instance will never call dealloc, as its 'sequence' property has a strong reference to it.

MIKMIDISequence.h:
@property (nonatomic, readonly, nullable) MIKMIDISequencer *sequencer;

MIKMIDISequence.m:
@interface MIKMIDISequence () { MIKMIDISequencer *_sequencer; }

MIKMIDISequencer.h
@property (nonatomic, strong) MIKMIDISequence *sequence;

[armadsen]

Fixed in master now, and will be in the 1.6.2 release.