initial ch07 commit
ex 7.1
ex 7.3 (books solution), 7.4
playing with Par
ex 7.5, 7.6
removed unnecessary type annotations
playing in ParWS
ex 7.11
ex 7.12 stub

ex 7.12, 7.13
7.12, 7.13 refined and tested in the worksheet
ex 7.14