Rewatch: detect changes to config and compiler version

[cknitt]

In case the rescript.json or the compiler version changes, rewatch should automatically clean before doing a build.

(Also, if the project was compiled with bsb before, do a clean before the build.)

[cknitt]

Like bsb does currently:

Different compiler version: clean current repo
Cleaning... 5 files.
Cleaning... 241 files.
Cleaning... 75 files.
Cleaning... 80 files.
...