The `PaymentHash`, `PaymentSecret`, `PaymentPreimage`, and
`ChannelId` types are all small wrappers around `[u8; 32]` and are
used throughout the codebase but were defined in the top-level
`ln/mod.rs` file and the relatively sparsely-populated
`ln/channel_id.rs` file.
Here we move them to a common `types` module and go ahead and
update all our in-crate `use` statements to refer to the new
module for bindings. We do, however, leave a `pub use` alias for
the old paths to avoid upgrade hassle for users.
We add the previously discussed `rustfmt.toml` and enforce it in CI for
any files that are not contained in an exclusion list.
To start, we add all current Rust files to this exclusion list. This
means that formatter rules will be enforced for any newly introduced
files, and we'll then start going through the codebase file-by-file,
removing them from the list as we go.
