Move `Payment{Hash,Preimage,Secret}` into a new crate

Move `Payment{Hash,Preimage,Secret}` into a new crate

`lightning-invoice` currently has a dependency on the entire
`lightning` crate just because it wants to use some of the useful
types from it. This is obviously backwards and leads to some
awkwardness like the BOLT 11 invoice signing API in the `lightning`
crate taking a `[u5]` rather than a `Bolt11Invoice`.

This is the first step towards fixing that - moving the common
types we need into a new `lightning-types` crate which both can
depend on.

Since we're using a new crate and can't depend on the existing
`lightning` hex utility to implement `Display`, we also take this
opportunity to switch to the new `Display` impl macro in
`hex_conservative`.