Merge: Warn call on nullable receiver

Followup of #1375 and #394

This introduce an advice on calls on a nullable receiver.
Since the conversion could be complex, I propose a light approach with advices (quiet warnings) instead of hard errors, so that:

1. jenkins can track them
2. nitpick (thus vim) can show them
3. mirgration can be done in an incremental and iterative way
4. issues with a strict call-on-nullable policy can be identified without breaking things
5. @Morriar can bank nitcoins in future PR

If (when?) all call on nullable can be removed, then the advice will become an error.

Pull-Request: #1414
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>

diff --cc src/model/model.nit
Simple merge

diff --cc src/semantize/typing.nit
Simple merge