Add solution to the 2021 midterm

previous-exams/2021-midterm/m21/project/MOOCSettings.scala

previous-exams/2021-midterm/m21/project/StudentTasks.scala

previous-exams/2021-midterm/m21/project/build.properties

previous-exams/2021-midterm/m21/project/buildSettings.sbt

previous-exams/2021-midterm/m21/project/plugins.sbt

...-midterm/m21/src/main/scala/m21/MultiWriterSeqCount.scala

...-exams/2021-midterm/m21/src/main/scala/m21/SeqCount.scala

...1/src/main/scala/m21/instrumentation/AtomicVariable.scala

...term/m21/src/main/scala/m21/instrumentation/Monitor.scala

...exams/2021-midterm/m21/src/test/scala/m21/TestSuite.scala

...21/src/test/scala/m21/instrumentation/MockedMonitor.scala

...rm/m21/src/test/scala/m21/instrumentation/Scheduler.scala

...idterm/m21/src/test/scala/m21/instrumentation/Stats.scala

...m/m21/src/test/scala/m21/instrumentation/TestHelper.scala

...rm/m21/src/test/scala/m21/instrumentation/TestUtils.scala

...exams/2021-midterm/m21/src/test/scala/m21/overrides.scala

previous-exams/2021-midterm/m3.md

previous-exams/2021-midterm/m3/.gitignore

previous-exams/2021-midterm/m3/assignment.sbt

previous-exams/2021-midterm/m3/build.sbt