Add coq-HoTT as a dependency for the .vo files

Some basic testing suggests that this will cause `make` to auto-re-coq
the .v files whenever you `git submodule update`, i.e., when it's
possible to tell that the version of Coq we're using has changed.
[transitivity] and [symmetry] use [_@_] and [_^]
Merge pull request #491 from JasonGross/trans-concat

Since @andrejbauer agrees, I am pulling this one.
Unfold [transitivity] and [symmetry] in the corresponding tactics
Merge pull request #464 from JasonGross/coq-HoTT-depend

Add coq-HoTT as a dependency for the .vo files
trunc_iff_isequiv_truncation via modalities
wrap funext into modalities to make coercions work
a few comments
identity modality
comments about the reason for repleteness