KMP support

[kirillzh]

@thunderbiscuit, the library is great! @jmateoac and I are starting to look at Cashu and would love to use the lib for a hackweek app. Is there any interest in migrating the library to KMP?

Going through dependencies, looks like we are halfway there.

Deps that are already KMP:

• org.jetbrains.kotlinx:kotlinx-serialization-json
• fr.acinq.bitcoin:bitcoin-kmp
• fr.acinq.lightning:lightning-kmp
• io.ktor:ktor-client-core

Deps that will need replacement:

• com.google.code.gson:gson -> org.jetbrains.kotlinx:kotlinx-serialization-json
  • https://github.com/thunderbiscuit/cashu-client/pull/9
• org.jetbrains.exposed:exposed-core, org.xerial:sqlite-jdbc -> SqlDelight is a good option here
  • https://github.com/thunderbiscuit/cashu-client/pull/11
• org.slf4j:slf4j-api, ch.qos.logback:logback-classic -> Kermit
  • https://github.com/thunderbiscuit/cashu-client/pull/10

We could migrate in small phases:

1. Replace JVM-only deps with KMP alternatives
2. Migrate lib to use KMP Gradle plugin, with JVM target to begin with
3. Add other targets, Android and Native would be a good start

[thunderbiscuit]

Hey that's awesome! I'm happy to migrate the JVM-only dependencies to KMP ones for sure, particularly if you're familiar with them and think they'd be a good replacement (I haven't worked with Kermit for example).

A few more thoughts:

• I'm super flexible on the architecture and happy to hear other developers thoughts. As this was developed one step at a time as I was reading the spec it's a bit all over the place. It's great if the lib can be looked at from fresh eyes.
• Just an FYI: you will find that the Cashu spec is moving super fast, and has accelerated over the past 6 months. I haven't touched the library much in that timeframe (the release of BDK 1.0 being my focus at the moment), and so it's now fairly outdated. If you're interested in bringing it up to spec go wild! I can try to review and merge fairly quickly if that's part of your hack week and you want to put time into updating it.

[kirillzh]

Branch with progress so far: https://github.com/kirillzh/cashu-client/pull/3