Since we have a simple way to query the database for UTXOs we can
simplify some of the coin selection logic. That gets rid of the
in-memory list of UTXOs.
Except for the trivial case of u8 arrays, have the generator create
the loop code for the array iteration.
This removes some trivial helpers, and avoids us having to write more.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
