Introduce KVStore trait and FilesystemStore impl

Rather than further relying on the upstream
`KVStorePersister`/`KVStoreUnpersister`, we here implement a general
`KVStore` trait that allows access to `Read`s/`TransactionalWrite`s
which may be used to deserialize/serialize data via the
`Readable`/`Writeable` implementations.

Notably `TransactionalWrite` is a `Write` for which the written data
needs to be explictly `commit`ed, asserting that we always persist
either the whole new change or no change at all.

Additionally, we avoid the `Info` umbrella term but opt to introduce `PaymentDetails`
to align naming with upcoming `PeerDetails` and `ChannelDetails`.

Author: tnull

Cargo.toml

@@ -66,6 +66,7 @@ libc = "0.2"
 electrsd = { version = "0.22.0", features = ["legacy", "esplora_a33e97e1", "bitcoind_23_0"] }
 electrum-client = "0.12.0"
 proptest = "1.0.0"
+regex = "1.5.6"
 
 [profile.release]
 panic = "abort"