WIP: preprocessor to handle file location in messages

This MR adds a preprocessor that replaces the LOC, FILE and LINE with the actuals locations in the file. It is useful to track bug, ie error that should not happen but still happen as we don't have a verified compiler yet.

Actual mechanism still have to be polished.

The current choice is to have a template file and not to update in-place in order to keep the original source file without having to deal with heavy build machinery. An other move could be to have a dune build, which would handle a kind of in-place preprocessing in the _build directory.