`bitcoin_tx_with_psbt` would somewhat opaquely steal the passed `psbt` value.
This caused a bug where code made a `bitcoin_tx` using a psbt without realizing the value was stolen. Because the resulting `bitcoin_tx` was placed in tmpctx it was not immediately clear that using `psbt` afterwards was an error until the tmpctx was cleared — creating a valgrind backtrace far from the actual issue.
Switching to the routine to using TAKES and adding documentation in the header, makes it explicitly clear which operation the user is doing — helping prevent future regressions of this kind.
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