Getting rid of casted_open_constr. It was only used by the
refine tactic, which now uses plain glob_constr's. Now there
is no real need to depend on goal when interpreting genargs.

Possible minor incompatibilities:

1. The interpretation of glob_constr to constr is now done by
Goal.constr_of_raw, which may be slightly dumbier than the dedicated
Tacinterp.interp_open_constr which tries harder. Stdlib and test-suite
do go through, though.

2. I had to change the parsing level of wit_glob in Extraargs
from lconstr to constr. It may break ML notations using glob, but
as they are only used inside Coq code and all well-parenthezised,
it should be OK.