Fix handling of unsimplified fractions in set & multiple answers

All of the new unit tests I added were things that were previously broken. :( Roughly the same bugs exist in Perseus too, so I guess I'll tackle that next...

Test Plan: Run unit tests at http://exercises.ka.local/utils/test/answer-types.html and try answering http://exercises.ka.local/exercises/completing_the_square_1.html?debug&problem=original with one part unsimplified and the rest otherwise correct, or with the rest wrong in some way.

Auditors: alex
Unrevert something I accidentally reverted because using git on a plane is slow and annoying

Test Plan: http://exercises.ka.local/utils/test/answer-types.html

Auditors: alex
review feedback; add moar tests

Test Plan: http://exercises.ka.local/utils/test/answer-types.html

Auditors: alex
Make problem more concise

There is no need to assume X is non zero.
Whether t is zero or not deosn't