Fixed an exception when there is more than one TempoChange at the same tick

See merge request gomidi/midi!7

sequencer: Close tracks in ToSMF1

Closes #37

See merge request gomidi/midi!5

in order to remove the only external dependency of gomidi/midi

gitlab.com/gomidi/smftrack

fix multiple includes, see https://stackoverflow.com/questions/56318343/golang-multiple-definition-of-cgo-ported-package