Make positivity check non-fatal (or disablable)

[catalin-hritcu]

I'm trying to resurrect the student's attempt at a normalization proof in F* but the positivity check gets in the way. I think the positivity check should be a non-fatal error, in the same class as being unable to discharge a proof obligation, but that's currently not the case. Alternatively I would like a flag to turn off the positivity check completely, so that I can fix the other potential errors that have nothing to do with positivity.

This is implemented in the positivity_check function in tc.fs, but I'm not sure with what to replace the raise to make it non-fatal.

[catalin-hritcu]

This issue is pre-universes. Now the positivity check is just gone (#319), so this issue no longer exists.