attempt to speed up proof of pmatch_to_i2_correct
strengthen statement of decs_to_i2_correct

have not yet proved the strengthened claimed, but have shown that it is
sufficient for prompt_to_i2_correct. the new conclusion says that if the
decs appear to have been from a module and we encounter an error, then
envC_tagged holds if we add the module constructor env anyway.
try proving the new decs_to_i2_correct

left 2 cheats that probably require the statement to be slightly
strengthened again: the situation is (I think) when a failing module
declares a constructor that duplicates a previous constructor; something
needs to keep track of the fact that this will somewhere be bound to the
module and hence doesn't actually override the old constructor. (might
be more subtle than what i imagine, though.)