Strengthen `tac_specialize_assert` to put the result in the intuitionistic context.

The result of `iSpecialize ("H" with "[H1 .. Hn]")` was always put in the
spatial context. This commit strenghtens this tactic by putting the result
in the intuitionistic context if the following conditions hold:

1. The hypothesis `H` is persistent.
2. All the hypotheses `H1 .. Hn` are intuitionistic (or similarly, in case
   `[-H1 .. Hn]` is used, if all remaining hypotheses are intuitionistic).
3. If the pattern `[> ..]` for adding a modality is not used.