This introduces various changes to the importmulti logic:
* Instead of processing input and importing things at the same time, first
process all input data and verify it, so no changes are made in case of
an error.
* Verify that no superfluous information is provided (no keys or scripts
that don't contribute to solvability in particular).
* Add way more sanity checks, by means of descending into all involved
scripts.
* Add release notes
This is a backport of Core PR14565