Define [f_ap] to replace the late [f_equal]

I'm not sure exactly how [f_ap] differs from [f_equal], as I've not
looked at the ml definition of [f_equal].  I've included a fairly
lengthy comment explaining how [f_ap] works, and included a note about
generalizing it.

This closes #139.
Make the issig tactics give better error messages
Merge pull request #141 from JasonGross/f_equal

Define [f_ap] to replace the late [f_equal]
Adding Section, explicit name to avoid
  Error: H is already used.
Merge branch 'master' of https://github.com/HoTT/HoTT
Merge pull request #144 from JasonGross/issig-error-messages

Make the issig tactics give better error messages