Solutions for 1.2-3, expand 1.1 solution.

This is a preliminary pull request adding solutions for the HoTT book exercises. I actually have much more formalized, but in a sort of haphazard formatting not appropriate for HoTTBook.v, so I want to first get some feedback on whether or not this is along the lines of what people would like to see in the exercises sheet. Are the answers too long? Non-idiomatic? How about the naming scheme for the subparts? If people like this, I'll prepare and submit more pull requests along these lines.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>