Scribble instrumentation check is too late

[cd1m0]

Scribble is checking if a file is already instrumented by looking for a marker at the start of the file. However that check is currently after the type-checking pass. Type-checking on an instrumented file however can already crash. We should move this check to right after source compilation.

[blitz-1306]

This is actually fixed by #220, that is already merged.