Moved local macros to `macros.tex`, but did a bit more work. #155

I got rid of `\zero`, `\one` and `\two` which were just aliases for
`\emptyt`, `\unit` and `\bool`, and also made some other minor changes.
It would be good if the authors (@kristinas, @mikeshulman) looked over it.
Moved macros to `macros.tex`, closing #155.

The rationale for this is that someone had to look at what all
the macros were doing anyway, so I did. The reason all macros
are in one place is that this way people will discover pre-existing
macros. I am not sure anymore that placing them all in `macros.tex`
was such a brilliant idea, but it probably does no great harm.
Merge branch 'master' of github.com:HoTT/book