support reading/writing PSBT from/to file

[instagibbs]

Would support simple interoperation with other wallets who only take them as files.

[jonathancross]

Related: python-trezor/#183 : Allow signing raw transaction in trezorctl, eg BIP-174 PSBT

[achow101]

@instagibbs Do you still want this for 1.0? I'm not sure that it is necessary to have this now. I don't think there is any software using PSBT files yet.

[instagibbs]

Glenn's work on top of Core does use it by default so I think it's something we should support Soon(TM). @Sjors re:1.0?

[achow101]

Glenn's work on top of Core does use it by default so I think it's something we should support Soon(TM).

Sure, but that won't be into a Core release until at least 0.19 so we still have time.

[instagibbs]

Fair enough. Move to 1.1/2 whatever versioning you desire.