Swap the dep order between `lightning` and `lightning-invoice`

Swap the dep order between `lightning` and `lightning-invoice`

`lightning-invoice` previously had 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`.

Here we finally rectify this issue, swapping the dependency order
and making `lightning` depend on `lightning-invoice` rather than
the other way around.

This moves various utilities which were in `lightning-invoice` but
relied on `lightning` payment types to make payments to where they
belong (the `lightning` crate), but doesn't bother with integrating
them well in their new home.