wollet: Update::prune keeps rangeproofs of outputs paying scripts introduced by the update

[ledegenbtc]

Update::prune() decides which outputs belong to the wallet by looking at wallet.cache.paths, but that map is only filled when an update is applied (apply_update_inner). With the documented usage, prune the update and then apply it (see the "Update Pruning" section of the book and tests/prune.rs), the scripts introduced by the update being pruned are not in the map yet, so their outputs are treated as foreign and lose the whole witness, rangeproof included.

This is invisible when the wallet already synced before receiving funds, because the gap-limit scripts are already in the cache: that is what test_prune does through TestWollet::new. It shows up when a funded script is first seen by the update being pruned:

• a wallet restored from its descriptor: the first full_scan carries the whole history and cache.paths is empty, so prune() behaves exactly like prune_witnesses(), contrary to its documentation;
• a wallet whose derivation index jumps ahead, e.g. many addresses handed out and funded before they are scanned.

The affected outputs then have no in_utxo_rangeproof when spent, and unblind_utxos_with() / reunblind() cannot see them.

Fix: Update::prune() also treats the scripts in scripts_with_blinding_pubkey as wallet scripts. DownloadTxResult::prune takes a predicate instead of a map, so the (possibly large) paths map is not cloned at every sync.

Tests:

• unit: test_update_prune_keeps_rangeproofs_of_new_scripts prunes the update_many_txs vector against a wallet that never synced and checks that the outputs paying the update's own scripts keep their rangeproofs, and that applying the pruned update yields the same utxos and balance. Fails on master.
• integration: test_prune gains a wallet restored from the descriptor after the transactions exist: prune, apply, build a PSET and check that in_utxo_rangeproof is present. Fails on master.

Relates to #158 (found while pruning updates in production; numbers in that thread).