Remove deprecated pa_labelscope camlp4 syntax extension

It is not compatible with the newest 4.02.0 OCaml version.
Update INSTALL
Manually flush stderr on logging

Apparently, OCaml 4.02 canged behavior and doesn't do automatic line buffering
for stderr.
Rename directories in test/
Shorten file names in src/
Update TODO
Add opam file for pinning
Remove tests dependency on environment

Remove no longer needed setenv.sh and bin/piqi