Update ocp-build binary to OCamlPro/typerex@c23ec435dcea2326ace4b88e506225e61bff7591
Fix Makefile after upgrade to the latest version
Continue 13fad278e657b44bd0fe05f97899e0cc2b24af9e

When we add --switch=SWITCH on the command line, we really want to see the environment variable for this switch (but not when we set $OPAMSWITCH).
Various improvement to the heuristic.

The current strategy (which seems to work fine) is:

1/ call the solver once to get a rought approximation of the final universe

2/ build a new universe where we:
   - install all the packages appearing in 1/; and
   - remove all the version below the one appearing in 1/

3/ use user-defined constraints to trim the universe (eg. for 'opam install core.109.15' we remove from the universe all the other versions of core)

4/ remove from 4/ all the versions of packages which are not a dependency appearing in the request (eg. it will remove every version of 'bin_prot' <> '109.15.00')

5/ brute-force explore the remaining universe to find a consistent state

This should fix #565 and #555
Do not fail on 'opam list' when no packages exist
Add ocp-build's colors back
export OPAMSWITCH only if using --switch=SWITCH
Use a different code-path when we want to remove packages.
Remove dead-code
Too long lines
Improve OpamCudf API
Update CHANGES