improved comments describing [eval_in]
Merge pull request #544 from mikeshulman/comment-evalin

improved comments describing [eval_in]
Solution to Exc. 3.9
Lemma 3.3.2,3
fixed misnomers in solution of Exc. 3.9
Added Book_3_3_2,3
simplified 3.9, Import HoTTBook can be a problem
reverted to d7fa7f...
Lemmas 3.3.2 and 3.3.3 added
Backrefs for Lemmas 3.3.2 and 3.3.3 added
Solution to Exc. 3.9
Many improvements in the solution of Exc.3.9