The functions inside of the crypto.go file in chanbackup (like EncryptPayloadToWriter and DecryptPayloadFromReader) can be used by a lot of things outside of just the chanbackup package. We can't just reference them directly from the chanbackup package because it's likely that it would generate circular dependencies. Therefore we need to move these functions into their own package to be referenced by chanbackup and whatever new functionality that needs them
In this commit, we introduce the Multi sturct. Multi is a series of
static channel backups. This type of backup can contains ALL the channel
backup state in a single packed blob. This is suitable for storing on
your file system, cloud storage, etc. Systems will be in place within
lnd to ensure that one can easily obtain the latest version of the Multi
for the node, and also that it will be kept up to date if channel state
changes.