Move main PSBT input logic to helper

This avoids the clone of the PSBT when doing the check.
Also fiddle with the checks that the checked function does a bit to make
it easier to explain.

Author: LLFourn

integration_test/src/test_desc.rs

@@ -13,7 +13,7 @@ use bitcoin::util::{psbt, sighash};
 use bitcoin::{self, Amount, OutPoint, SchnorrSig, Script, Transaction, TxIn, TxOut, Txid};
 use bitcoincore_rpc::{json, Client, RpcApi};
 use miniscript::miniscript::iter;
-use miniscript::psbt::PsbtExt;
+use miniscript::psbt::PsbtInputExt;
 use miniscript::{Descriptor, DescriptorTrait, Miniscript, ToPublicKey};
 use miniscript::{MiniscriptKey, ScriptContext};
 use std::collections::BTreeMap;