Happy pi day! Finally add support for mixed number multples of pi

fix #43256; fix #43249

Test plan:
http://localhost:8000/exercises/circles_and_arcs.html?debug&problem=r-angle-to-a&seed=122&activity=%5B%5B%22incorrect-activity%22%2C%22%5C%221%201%2F4%20pi%5C%22%22%2C3%5D%2C%5B%22incorrect-activity%22%2C%22%5C%224%205%2F18%20pi%5C%22%22%2C7%5D%2C%5B%22incorrect-activity%22%2C%22%5C%222%205%2F36%20tau%5C%22%22%2C80%5D%2C%5B%22correct-activity%22%2C%22%5C%2277%2F36%20tau%5C%22%22%2C14%5D%5D

Auditors: alpert