Overhaul importmulti logic

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.

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