Support BOLT#11 fallback

[joshmh]

It would be really useful for all the lightning mobile wallets to support the fallback mode described in BOLT#11. This means that if the wallet comes across an old BIP21 URI, it will parse it as it does now, then look for a lighting parameter. If it finds it, it just takes that parameter and treats it as a regular lightning invoice. If not, it treats the URI as a regular Bitcoin URI.

This is especially important for Bitcoin ATMs. We can show a single QR code for everyone. If the user has a lightning wallet, they can send the funds instantly (and receive cash on the spot). If the user has an old Bitcoin-only wallet (currently the majority), they can just send Bitcoin. Eventually, lightning wallets could use the Bitcoin address to send large payments for which a path can't be found.

SwiftLightning/ios/SwiftLightning/Models/Lightning Shim/LNManager.swift

Line 182 in 81aa62a

else if inputString.hasPrefix(PayReqPrefixes.btcuri.rawValue) {

See: ACINQ/eclair-mobile@56dd9c5

https://twitter.com/meeDamian/status/1019505368760659968