Very close to a working simulfund signer!
Just broadcast first simulfund tx!
Full coin roundtrip, through LB create, simulfund, spend
Merge branch 'simulfund' into devel

Alpha simulfunding -- it works!  Still limitations, but it works for
the most common use cases, which is simultaneously funding a single
lockbox from a series of single-sig wallets.  It should be pretty easy
to expand this to do arbitrary CoinJoins, as well simulfunding with
other lockboxes or strange scripts.