Add PERL5LIB to `opam config env`

Some opam packages (e.g. CIL) need to install perl helper scripts.
To follow perl conventions while respecting the user's expectation
that opam does not install anything outside of `opam config root`,
these scripts are installed in `opam config var lib`/perl5.

This patch modifies `opam config env` to add this directory to the
user's PERL5LIB environment variable, which allows him to use the
installed scripts seamlessly. Note that it's not an issue to set
this variable even if the directory does not exist (e.g. if no
package using perl has been installed), since perl will simply ignore
the non-existing directory.
Merge pull request #755 from kerneis/master

Add PERL5LIB to `opam config env`
Merge pull request #764 from vbmithr/master

Typo.
fix opam remove
Merge pull request #769 from hhugo/master

fix opam remove
Cache the repository priority and the corresponding prefixes regarding packages and compilers

compiler index is in ~/.opam/repo/compiler-index
package index is in ~/.opam/repo/package-index

~/.opam/repo/index is now deprecated, and editing this file to modify package priority manually is not supported anymore. Nobody was using it and it was quite a pain to maintain for no good particular reasons.
Experimental extra-fast init/update

We keep in the global state only the metadata of installed packages. So ne need to have a costly synchronization on each update.

Very untested, but at least tested on 'opam init --no-setup && opam list` wich becomes increadibly fast. Need to think a little more about the corner cases now ...
Read the cached indexes instead of computing them on every opam invocation
Add some timers to find hotspots more easily
Change the global stats thing

Now we just print the list of read/write structured files
Do not evaluate the format string when debug is not set
Add stats about external command calls
Fixing pinned packages: only the pinned version is 'available'
Changed 'installed' files to contain 'pinned' for pinned packages
that way, you can change the version in the package's .opam file without breaking everything
Fixed descrs for pinned packages
(and full-descrs for all of them :p)
Fixed conversion `pinned` <-> `real_version` in opamCudf

Closes #744
Fixed unmarking of packages as roots after reinstall (closes #759)
Merge pull request #751 from AltGr/fix-pin

Fixing pinned packages #1: only the pinned version is 'available'
Fixed resolution of path in `opam repo add`
Closes #763
Fixed resolution of package/repo names for update
Closes #748