PackageTemplate aims to replace InputMaterial, introducing a clean
interface to manipulate a wide range of claim types without
OnchainTxHandler aware of special content of each.
This is used in next commits.
Package.rs aims to gather interfaces to communicate between
onchain channel transactions parser (ChannelMonitor) and outputs
claiming logic (OnchainTxHandler). These interfaces are data
structures, generated per-case by ChannelMonitor and consumed
blindly by OnchainTxHandler.