Add some scripts to automate performance tracking

To get the performance of the current library, run

$ ./etc/timing/make-pretty-timed.sh

This prints out a log, and puts it in `time-of-build-pretty.log`.  You
can pass any arguments to this that you would pass to `make`.

To get the performance of your current change, and then commit it with a
performance log, run

$ git status
$ git add <all files mentioned above that you care about tracking>
$ ./etc/timing/make-pretty-timed-diff.sh
$ git commit -at ./time-of-build-both.log

You should update the commit message to have a title and body, and not
just a performance log.

Currently, the build-time of this library on MIT's cagnode machines is:

Time     | File Name
-----------------------------------------------------------------
2m00.84s | Total
-----------------------------------------------------------------
0m20.22s | categories/Functor/Paths
0m14.46s | hit/Flattening
0m09.61s | categories/GroupoidCategory/Morphisms
0m08.58s | hit/Connectedness
0m07.07s | categories/Functor/Sum
0m06.87s | categories/Category/Morphisms
0m06.06s | EquivalenceVarieties
0m04.52s | types/ObjectClassifier
0m03.20s | types/Sigma
0m03.16s | Equivalences
0m02.57s | categories/Category/Sum
0m02.48s | categories/NaturalTransformation/Duals
0m02.38s | PathGroupoids
0m01.86s | categories/NaturalTransformation/Paths
0m01.82s | categories/NaturalTransformation/CompositionLaws
0m01.76s | categories/FunctorCategory/Morphisms
0m01.52s | categories/Category/Prod
0m01.48s | types/Prod
0m01.41s | categories/NaturalTransformation/Composition
0m01.04s | hit/Spheres
0m00.99s | types/Paths
0m00.97s | hit/epi
0m00.96s | Trunc
0m00.72s | categories/InitialTerminalCategory
0m00.66s | types/Sum
0m00.66s | Functorish
0m00.63s | hit/quotient
0m00.59s | hit/Circle
0m00.55s | hit/Suspension
0m00.49s | categories/NaturalTransformation/Sum
0m00.48s | types/Record
0m00.46s | categories/Functor/CompositionLaws
0m00.46s | HProp
0m00.46s | types/Forall
0m00.44s | types/Universe
0m00.41s | HSet
0m00.40s | Fibrations
0m00.34s | Tactics
0m00.34s | categories/IndiscreteCategory
0m00.34s | TruncType
0m00.30s | hit/minus1Trunc
0m00.30s | categories/Functor/Duals
0m00.28s | types/Arrow
0m00.26s | categories/NaturalTransformation/Identity
0m00.26s | Misc
0m00.24s | Conjugation
0m00.24s | categories/Category/Objects
0m00.22s | categories/NatCategory
0m00.22s | categories/Category/Category
0m00.19s | FunextVarieties
0m00.19s | categories/Functor/Composition
0m00.19s | categories/FunctorCategory/Core
0m00.18s | categories/categories
0m00.16s | categories/GroupoidCategory/Core
0m00.16s | hit/unique_choice
0m00.15s | categories/NaturalTransformation/Core
0m00.15s | categories/Notations
0m00.14s | categories/Category/Notations
0m00.14s | coq/Init/Peano
0m00.14s | Overture
0m00.14s | categories/Category/Univalent
0m00.14s | categories/Utf8
0m00.13s | categories/Category/Duals
0m00.13s | hit/Truncations
0m00.12s | categories/Category/Utf8
0m00.12s | categories/NaturalTransformation/NaturalTransformation
0m00.12s | Contractible
0m00.12s | hit/Interval
0m00.12s | categories/Category/Core
0m00.12s | types/Bool
0m00.11s | categories/FunctorCategory/Utf8
0m00.11s | categories/Functor/Utf8
0m00.11s | categories/FunctorCategory/FunctorCategory
0m00.10s | categories/DiscreteCategory
0m00.10s | categories/Functor/Functor
0m00.10s | categories/Functor/Notations
0m00.09s | categories/GroupoidCategory/GroupoidCategory
0m00.09s | categories/Functor/Identity
0m00.09s | categories/Functor/Core
0m00.08s | HoTT
0m00.08s | types/Unit
0m00.08s | categories/NaturalTransformation/Notations
0m00.08s | coq/Init/Logic_Type
0m00.08s | UnivalenceAxiom
0m00.08s | categories/NaturalTransformation/Utf8
0m00.08s | categories/FunctorCategory/Notations
0m00.08s | types/Empty
0m00.07s | coq/Init/Specif
0m00.07s | FunextAxiom
0m00.06s | coq/Init/Datatypes
0m00.06s | categories/Category/Strict
0m00.06s | coq/Program/Tactics
0m00.04s | coq/Init/Notations
0m00.04s | coq/Init/Prelude
0m00.04s | coq/Bool/Bool
0m00.04s | coq/Init/Tactics
0m00.04s | coq/Init/Logic
Quiet Makefile (especially make TIMED=1)

Before, we displayed things like

"/usr/bin/time" -f "coq/theories/Program/Tactics (user: %U mem: %M ko)" /afs/csail.mit.edu/u/j/jgross/.local64/bin/coqtop -indices-matter -boot -nois -coqlib ./coq -R ./coq/theories Coq -compile coq/theories/Program/Tactics

Now we display

COQTOP coq/theories/Program/Tactics

The old behavior can be obtained by passing V=1.
Use the pretty timing on travis

This way, we have a relatively easy way of seeing how slow files are.

We also have to install `time` on travis.
Opacify path_functor'_sig
Have travis fail if the build fails

I had forgotten that make-pretty-timed doesn't fail just because the
library fails to build.  This way, we still see a pretty table of the
performance, but also know when the build fails.
Document timing scripts
Update readme to mention timing scripts.