Move `[u8; 32]` wrapper types to a common module

Move `[u8; 32]` wrapper types to a common module

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.