Fix the implementation of Support.{remove_,}extension and update tests

[dinosaure]

A simple fix for #16 where I decided to took the implementation from OCaml (and add the copyright). The support module depends on Sys now and the patch seems to modify the order of modules into the module_collision tests.

[dinosaure]

I think this PR is ready. I added a documentation to precise that the semantic of extension is different than Filename.extension from the standard library. The current implementation of codept expects the extension without the dot when Filename.extension keeps the dot.

[Octachron]

The testsuite should be fixed now and the first commit can be dropped.

[dinosaure]

The testsuite should be fixed now and the first commit can be dropped.

Done :+1:.